The head of the Belarusian state security service warned today that protesters who take to the streets on election day could be charged with terrorism and executed.

Stepan Sukhorenko, the KGB chief, also accused other nations of involvement in opposition plots to seize power in a coup during the election, which takes place on Sunday.

"We will not allow the seizure of power under the guise of presidential elections," Mr Sukhorenko told a news conference. "For those who take the risk of going out into the street and try to destabilise the situation, their actions will be qualified as terrorism."

Demonstrators who "try and destabilise the situation will be charged with terrorism" and face sentences of up to 25 years, or life in prison, or capital punishment, he said.

Mr Sukhorenko’s announcement was similar to previous statements that the security services had uncovered Western-backed plots to destabilise the former Soviet state of 10 million people, which has been accused in the West of crushing human rights.

This month, the security chief went on television to announce that activists had been plotting explosions ahead of the poll.

But the timing of the latest statement and stark warning to demonstrators added high drama to the election, in which Alexander Lukashenko, the country's authoritarian President, is universally expected to beat his two rivals from the liberal and nationalist opposition.

Mr Sukhorenko showed reporters footage of an unidentified young man saying that he had undergone military training at a camp in Georgia. "Brigades of volunteers are being formed in neighbouring states," he said."The key moment will be detonation of several explosions."

Several key figures in the opposition camp have been rounded up in recent weeks and fined or sentenced to short prison terms.

Western countries have threatened to toughen sanctions against Belarus if, as expected, independent observers declare the election neither fair nor free.

Alexander Milinkevich, the main opposition candidate, has urged supporters to mass at polling stations to keep the count honest, but the government ban on rallies Sunday has potentially set the stage for violent confrontation.

Mr Lukashenko, in power since 1994, has said that he will tolerate no upheaval like the mass protests that helped to bring liberals to power in Georgia and Ukraine, two other ex-Soviet states.

Adrian Severin, the UN special investigator on human rights in Belarus, condemned Mr Lukashenko's Government for its attacks on the independent media, opposition parties and rights advocates.

Authorities should "immediately cease all actions aimed at intimidating opponents, journalists and human rights defenders and jeopardising the free and fair running of the electoral campaign," Mr Severin said in a statement.

Mr Lukashenko has ruled with an iron fist since 1994, quashing dissent and maintaining his grip on power through elections dismissed as illegitimate by critics. He is seeking five more years at the helm after pushing through a referendum scrapping term limits.

Belarusian authorities stepped up the pressure on opposition groups yesterday by detaining Anatoly Lebedko, a party leader who has thrown his weight behind Mr Milinkevich, and barring three members of an international monitoring mission from the country.

Mr Severin urged Belarusian authorities "to release immediately and unconditionally all political opponents, to bring all violations of freedom of expression and of the right of peaceful assembly to an immediate end, and to ensure that the elections are held in full compliance with international standards".

He expressed concern for another opposition candidate, Alexander Kozulin, who he said had been detained and beaten. He also condemned the imprisonment of some of Mr Milinkevich’s top aides and other political activists.

Mr Severin prepares regular reports on the situation in Belarus for the UN Human Rights Commission, but receives no co-operation from the Government and is barred from entering the country.