SAN FRANCISCO (Reuters) - The Vietnamese government restricted access to Facebook Inc inside Vietnam for several days this week as part of a broader crackdown on human rights and political dissidents during a visit by President Barack Obama, two activist organizations said on Thursday.  Officials of Access Now, a digital rights organization, and Viet Tan, a Vietnamese pro-democracy group, said the social media site was restricted and at times blocked inside Vietnam from Sunday to Wednesday, citing reports from people inside the country on Twitter and to Access Now’s digital security help service.  The move coincides with a trend toward restrictions on Facebook in countries including China, Uganda and Turkey during politically sensitive times as the 1.6 billion-person social network grows more powerful.  Obama’s three-day visit to Vietnam ended on Wednesday. Obama largely focused on normalizing relations with Vietnam. But he also promoted human rights and chided Vietnam about restrictions on political freedoms after critics of its communist-run government were prevented from meeting him. The Facebook shutdown was part of a stepped-up campaign by the Vietnamese government to limit use of the social network for political protests, activists said in phone interviews.  Facebook was blocked several times earlier this month as street protests erupted over an environmental disaster that resulted in mass fish deaths, the two groups said. The social media site was also unavailable inside Vietnam ahead of parliamentary elections on Sunday as pro-democracy activists called for a boycott, members of the two groups said. Facebook declined to comment. Vietnamese government officials did not immediately respond to a request for comment submitted via a government website.  Uganda’s government blocked Facebook and Twitter Inc in February during presidential elections. In March, after a deadly bombing in Turkey, an Ankara court ordered a ban on access to Facebook and Twitter. And during the 2011 Arab Spring in Egypt, Tunisia and Libya, social networks were repeatedly shut down.  Facebook is often shut down in Vietnam during politically sensitive times, Angelina Huynh, advocacy director for Viet Tan, which has members around the world, including in Vietnam, said in a phone interview.  “People were using Facebook to call for protests. They did not want people to take to the streets,” Huynh said.  