MOSCOW - Prime Minister Vladimir Putin strongly warned his opponents against unsactioned protests after Sunday's presidential election, in which he is all but certain to regain the presidency.

In a statement reflecting heightening tensions four days before the vote, he also alleged Wednesday that his foes may kill a prominent opposition figure in order to fuel public outrage against the government.

"They are looking among well-known people for a sacrificial victim," he said, according to Russian news reports. "They could, I'm sorry, knock someone off and then blame the authorities for that."

Putin criticized the opposition plans for rallies over what it fears will be a fraudulent election, saying Wednesday it is "unacceptable" to prejudge the vote.

"We will respect any viewpoint but are calling on everyone to act within the framework of law and use only legitimate means," he said at a meeting with his campaign activists.

Evidence of widespread vote-rigging in favour of Putin's party in December's parliamentary election fueled a series of massive protests in Moscow demanding an end to Putin's 12-year rule. Their organizers had received the authorities' clearance in advance to avoid any violence.

Civil society and opposition activists have turned out en masse to act as observers at the polls to prevent violations. They are also bracing up for demonstrations after the vote.

The opposition is now pushing authorities to allow a postelection protest at a venue even closer to the Kremlin. The Moscow city government has refused the demand, offering other more distant locations instead. The organizers have rejected that offer, raising the threat of violence.

Putin claimed Wednesday that unidentified forces abroad are plotting provocations against authorities after the vote. He has repeatedly claimed that the U.S. is stoking up protests in order to weaken Russia.

Putin, who was Russia's president from 2000 to 2008 and has been prime minister since then, is running for a third, now six-year presidential term. Polls have showed he is likely to easily defeat four Kremlin-approved challengers, but his statements Wednesday reflected strong concern about the opposition protests.

Previous rallies in Moscow that drew tens of thousands in the largest show of discontent since the Soviet times were sanctioned by authorities and went on peacefully.

Putin seems to command the strong loyalty of police and other law enforcement agencies, which have seen significant wage increases recently. But a violent confrontation after Sunday's vote could fuel public anger against Putin and destabilize the situation.