A bomb hidden inside a motorcycle exploded in front of the offices of a Muslim scholars committee in Arsal, the official National News Agency said.
The Sunni clerics are believed to have been in trying to negotiate the release of Lebanese security personnel held by jihadist militants based in Syria.
Arsal was the scene of fierce fighting between the two sides last year.
The Sunni town, which hosts many Syrian refugees, is sandwiched between Syrian government-held territory and predominantly Shia Lebanese areas sympathetic to it.
The NNA reported that the head of the Qalamoun Clerics Association, Sheikh Othman Mansour, was among those people killed by Thursday's blast in Arsal.
The agency said the bomb was placed on a motorcycle that blew up outside the association's office.
But a security source told the AFP news agency that the bomber had entered the offices during a meeting of the clerics and detonated an explosive belt.
"The explosion definitely targeted this meeting... where usually no less than 15 people are gathered," Arsal resident Abu Ibrahim told AFP.
The scholars were involved in trying to negotiate the release of more than two dozen members of the Lebanese security forces who were seized by al-Qaeda's affiliate in Syria, al-Nusra Front, and Islamic State militants during a major attack on Arsal in August 2014.
The jihadist groups have demanded that the Lebanese authorities free Islamist prisoners in exchange, and have killed four of the captives in an attempt to force them to act.
The conflict in Syria has heightened sectarian tensions in Lebanon, with violence regularly spilling over its border and more than a million refugees arriving.

Lyth hit two sixes and 13 fours in his 116 not out and shared a third-wicket stand of 150 with skipper Andrew Gale.
Gale's 61 was his highest score of the season, but he fell to Tom Curran as his side closed on 207-3, 60 behind.
Earlier on day three, Sam Curran (59*) and Steven Davies (56) helped Surrey add 103 to their overnight 164-5.
With the best part of four sessions lost to rain on the first two days, a draw is now the most likely result, but having begun the game in fifth place, Yorkshire will have set their sights on reaching 400 to ensure they leave the Oval with maximum bonus points.
After Davies and the younger Curran added 75 for the seventh wicket, Surrey lost their last four for 28 in the space of 10 overs.
Alex Lees departed in the eighth over of Yorkshire's innings and Jack Leaning followed for a duck when he chopped Stuart Meaker's first delivery into his stumps.
Lyth, though, was unperturbed as he progressed to three figures off 159 balls and partner Gale, who started the match averaging only 17.93, grew in confidence to reach his first half-century of the summer before he was caught behind.
Surrey all-rounder Sam Curran told BBC Radio London:
"It was nice to finally get back into the side after school and stuff, to get some time in the middle.
"They showed they are one of the top teams in the division and I had a good battle with Adil Rashid, a good spinner, he really made me work on my feet.
"It's been frustrating. Both teams wanted four days (of play) because we're both looking for wins.
"Tomorrow we're going to have to come in quite hard with the ball, see how many wickets we can take in the first session and see where we can take it from there."
At least six people are reported to have been killed in a bomb attack in a town near Lebanon's border with Syria.
[NEXT_CONCEPT]
Yorkshire opener Adam Lyth made his third century of the season as the defending champions replied solidly to Surrey's 267 all out at the Oval.