Liberian police Wednesday denied there was ever any attempt to assassinate presidential hopeful Winston Tubman at a rally on the eve of the run-off vote.

Tubman, who cried foul after trailing incumbent Ellen Johnson Sirleaf in the election's first round last month and urged voters to boycott Tuesday's run-off, charged that a deadly shooting at a rally Monday was an assassination bid.

But the police chief denied that there ever any plan to assassinate Tubman during the incident, which left at least four opposition supporters dead.

"You would have to be monumentally stupid to make an attempt to assassinate not only Mr Tubman, but any political figure. It wasn't the plan then, now or tomorrow," Inspector General Marc Amblard told AFP.

"I categorically deny there was ever an attempt to assassinate him."

Opposition supporters gathered in front of Tubman's party headquarters on Monday but his planned march was not authorised and tension between protestors and riot police erupted into violence.

"I was getting out of the car, a young man pushed me back and ... a bullet that we believe was aimed at me by a sniper hit him instead of me, and he died," Tubman told journalists.

"I am of the belief that they (police) were acting under orders, and those orders indicated they were ordered to eliminate me. I was a target of sniper fire," he said.

Amblard said the police had yet to see any bodies, and Tubman's Congress for Democratic Change would not allow them access to their headquarters to investigate.

"If Mr Tubman has any concerns he should allow law enforcement agencies to carry out their work," Amblard said, adding that his remarks were "very dangerous and very arrogant."

Meanwhile one policeman was injured on Tuesday night in New Kru Town when a group of youth stoned National Electoral Commission officials attempting to retrieve ballot boxes.

Amblard said police responded and one was injured and taken to hospital, but the ballot box was safely retrieved.

Vote counting was under way Wednesday, with Sirlead poised for reelection after a run-off marked by low turnout.