CAIRO—Egyptian President Hosni Mubarak handed power to his vice president but retained his title, a half-measure that confused observers, angered opposition leaders and provoked an uproar from hundreds of thousands of protesters massed in the center of the country's capital. 
 
 Charles Levinson has the latest from Cairo where protesters are reacting to President Mubarak's decision to remain in power. John Bussey and Robert Danin look at what's next for Egypt and U.S. relations with that country. 
 
 Hosni Mubarak surprised many when he announced late today he would not step down as Egypt's President until elections in September. Tamer El-Ghobashy has reaction from Cairo's Tahrir Square. John Bussey and Jerry Seib have analysis of the situation. 
 
 The move sets up another conflict with the opposition movement, which has called on supporters to gather for a huge protest Friday. Expectations that the president would resign had built through the day, and the immediate reaction to the speech was anger, with protesters chanting "Leave, leave." 
 
 An Army officer using a loudspeaker tried to calm protesters. "Let's save our energy for tomorrow," a man screamed to the crowd. "Go home and sleep, because tomorrow will be the day of judgment." 
 
 The defiant tone taken by Mr. Mubarak—and widespread confusion about the meaning of his speech—left the Obama administration scrambling to devise its next steps in a crisis that appears out of its control. 
 
 After Mr. Mubarak's speech, President Barack Obama, in a statement from the White House, said "The Egyptian people have been told that there was a transition of authority, but it is not yet clear that this transition is immediate, meaningful or sufficient." 
 
 Mr. Obama called on the Egyptian government "to move swiftly to explain the changes that have been made." 
 
 Mr. Mubarak's speech left the White House with a sense of "disbelief," a U.S. official said. 
 
 Egyptian officials said Mr. Mubarak gave the U.S. much of what Vice President Joe Biden requested earlier in the week: the delegation of presidential powers to the vice president, Omar Suleiman. 
 
 But Mr. Suleiman, who delivered a televised address after Mr. Mubarak, enraged protesters by refusing to distance himself from the president, even as he gestured toward the legitimacy of the protesters' demands. 
 
 The reaction casts a shadow over hopes by the U.S. and other traditional Egyptian allies that Mr. Suleiman would become a figure who is able to orchestrate a smooth transition from Mr. Mubarak to a more open electoral process in September. 
 
 The White House is now squeezed between Arab and Israeli allies, who have complained loudly that Mr. Obama was pushing Mr. Mubarak too hard to step down, and lawmakers who accuse the White House of not pushing hard enough. Now, the White House finds itself largely a bystander. 
 
 "This is really bad," a senior U.S. official said after Mr. Mubarak's address. "We need to push harder—if not, the protests will get violent." 
 
 Mr. Mubarak's language and refusal to yield to what he called the intervention of foreigners—a reference that appeared directed primarily at the U.S.—left protesters furious and the situation in Cairo precarious. 
 
 The anger welling among protesters placed the country's army in a tight position in its quest to keep the nation stable and secure. 
 
 Opposition leader Mohamed ElBaradei, after Mr. Mubarak's address, told CNN that the president and Mr. Suleiman were "twins" and that "neither are acceptable to the people." 
 
 In a earlier post on Twitter, Mr. ElBaradei wrote: "Egypt will explode. Army must save the country now." 
 
 Mr. Mubarak's speech continued a pattern of making concessions short of what protesters have been demanding. "I've seen fit to delegate presidential powers to the vice president, as according to the constitution," Mr. Mubarak said in his speech. 
 
 But he said he was "adamant to continue to shoulder my responsibility to protect the constitution and safeguard the interests of the people." He vowed that he would remain in the country and said he was addressing the youth in Tahrir as "the president of the republic." 
 
 Israeli analysts remain concerned about possible new threats to the country's security amid unrest in Egypt and elsewhere in the Middle East. Special correspondent Martin Himel reports from Tel Aviv. 
 
 Egyptian Ambassador to the U.S. Sameh Shoukry told CNN that Mr. Mubarak had passed all powers of the presidency to Mr. Suleiman, except for three: to amend the constitution, dissolve parliament—stacked with supporters of Mr. Mubarak—and fire the cabinet. 
 
 Those powers now belong to no one, Mr. Shoukry said, insisting that Mr. Suleiman was now the "de facto" president. 
 
 That interpretation wasn't immediately clear from the speech, however, and its nuances appeared lost, or irrelevant, to opposition leaders and protesters waiting to hear the president say he was quitting. 
 
 Instead, the president's comments brought a deflating end to a day of rumors and contradictions. 
 
 Hossam Badrawy, secretary general of the ruling National Democratic Party, had said early in the day that he told Mr. Mubarak that the president needs to step down for the good of the country, and said he believed the president would go. 
 
 Egypt's powerful military signaled it was moving to take control of the country. State television showed the Supreme Council of the Armed Forces meeting—conspicuously without Mr. Mubarak present. 
 
 The armed forces "have started taking necessary measures to protect the nation and support the legitimate demands of the citizens," an army spokesman said on state television. 
 
 The meeting and statement that followed appeared to represent a break from the military's previous posture as a neutral party providing security. 
 
 Those developments drew what was possibly the biggest crowd yet to Tahrir Square, the epicenter of protests that began Jan. 25. 
 
 Western officials, senior Egyptian government officials, and opposition leaders appeared to believe it was likely that Mr. Mubarak would announce he was stepping down and that the military would take over. 
 
 WSJ's Tamer El-Ghobashy reports from the streets of Cairo on demonstrations expanded beyond Tahrir Square, and that government workers are striking in opposition to president Mubarak refusing to step down. 
 
 Google Inc. executive and Egyptian protest leader Wael Ghonim said in his Twitter account: "Revolution 2.0: Mission Accomplished." 
 
 Question marks hang over what transpired inside the regime's corridors of power between early evening in Cairo, when signs pointed to Mr. Mubarak's stepping aside, and 10:50 p.m., when the president gave his televised address. A person familiar with Egypt's power centers said a possible wild card in Mr. Mubarak's decision could be the presidential Republican Guard, the opaque force that is responsible for the president's personal security. 
 
 When Mr. Mubarak's face appeared on a makeshift television screen made from a large sheet in one corner of the square, the crowd suddenly hushed for what many thought would be a momentous event: the resignation of their president for 29 years. 
 
 The crowd formed clusters, around people with radios and cellphones. People listened impatiently, and expressed frustration at the wordiness of his speech. As the end of the speech neared, a groan was audible in the masses. Some began chanting "resign" only to be told to shut up by those still listening. 
 
 As it became increasing clear Mr. Mubarak wasn't resigning, many in the crowd began to hiss. 
 
 "I don't like what I'm hearing," said Mohamed Mustafa, 49 years old, huddling with four others around a small radio. 
 
 Once it was over, people joined in a massive chant of, "Resign means leave, obviously you don't understand!" 
 
 Soldiers who had abandoned their posts outside ran back to the edges of the square and eventually to their positions near tanks. 
 
 "This will make people go mad," said Sherif Oman, a 24-year-old engineer. "He will regret this speech tomorrow. He is insulting us with this speech." 
 
 After a period of spirited chanting, the stunned crowd made its slow exodus from Tahrir Square. Some protesters marched on the nearby state television building; most appeared to opt for home. 
 
 Some protesters meanwhile began to chant, "Tomorrow we go to the palace!" 
 
 —Matt Bradley, Sam Dagher and Christopher Rhoads contributed to this article. 
 
 Write to Margaret Coker at margaret.coker@wsj.com, Charles Levinson at charles.levinson@wsj.com and Jonathan Weisman at jonathan.weisman@wsj.com ||||| Mohamed ElBaradei, who has emerged as a leader of the opposition movement against President Hosni Mubarak, posted comments online within the last hour warning that the country has become volatile. 
 
 “Egypt will explode," ElBaradei, a Nobel Peace Prize winner and retired diplomat, posted on Twitter. "Army must save the country now." 
 
 In statements posted earlier in the day, ElBaradei vowed continued protests and complained of violence by Mubarak's government.
– One of the first public reactions to Hosni Mubarak's non-resignation doesn't bode well for what happens next: “Egypt will explode," wrote Nobel laureate and pro-democracy advocate Mohamed ElBaradei on Twitter, reports the Los Angeles Times. "The Army must save the country now. I call on the Egyptian army to immediately interfere to rescue Egypt. The credibility of the army is on the line." In Tahrir Square, protesters are seething and calling for bigger demonstrations tomorrow, notes the Wall Street Journal.