By Olena Horodetska

MINSK, Mar 18 (Reuters) The Belarussian opposition, reaching the last stage of its near-hopeless attempt to unseat President Alexander Lukashenko, urged supporters today to mass in the streets to ensure a free election.

Opposition challenger Alexander Milinkevich accused Lukashenko of preparing to rig Sunday's presidential vote while the European Union, which has sanctions against top Belarus officials, expressed concern over a ''lack of freedom''.

In words reminiscent of the 2004 revolution in neighbouring Ukraine, Milinkevich told a small rally of supporters to take to the streets after the polls closed, despite official warnings that violations of public order would be treated as terrorism.

''I had expected the campaign to be unfair and dishonest but I had not expected it to be so outrageous. We want to tell the truth about it. I invite you to come to the square,'' he told about 200 pensioners and students.

Shopkeepers and waiters half-whispered the phrase ''come to the square'' to passersby on the streets of the capital, Minsk, which were full of police and soldiers.

Anonymous leaflets called for revolution, and mobile phones received competing text messages alternately encouraging protests or warning people against falling victim to bloodshed.

''We are going there with flowers, peacefully. We are against violence. We are going to the square to say no to falsification,'' said Milinkevich.

Opposition activists planned a concert for later in the day.

Lukashenko, accused by the opposition and the West of oppressing rivals and rigging polls since the mid-1990s, is all but certain to win against three challengers on Sunday.

''WRING THE NECKS'' He vows to crush any upheaval like the post-election protests that unseated the Moscow-backed establishments in Georgia and Ukraine and to ''wring the necks'' of rivals who dare to violate public order.

European Commission President Jose Manuel Barroso warned Lukashenko that more harassment of the opposition would further damage Belarus' ties with the European Union, which has already imposed a travel ban on Lukashenko and other top officials.

''We are very concerned about some developments that we have seen recently, namely the lack of freedom, the harassment of the leaders of the opposition and other very negative developments,'' he told reporters during a visit to Moscow.

Dozens of opposition activists have been fined and sentenced to short jail terms for public order offences. International observers and reporters have been denied entry or deported, while independent newspapers have been seized.

Lidyia Yermoshina, head of the Central Electoral Commission, said campaigning was fair and equal for all candidates and that attempts to pressure Lukashenko would backfire.

''I think our people will unite even more as a result and demonstrate their best quality -- love of their motherland,'' she was quoted by the state news agency BelTA as saying at a meeting with international observers.

She said about 20 percent of voters had already cast their ballots during advance voting -- a practice criticised by the opposition as a vehicle for fraud.

The opposition denounced leaflets promising revolution as an attempt to blacken its image, while voters were confused by contradictory text messages of unknown origin.

''Freedom is close as never before. We are a majority. Come to the square,'' said one.

But another told a different story.

''Bloodshed is being prepared on the square on March 19. Take care of your health and life,'' it said.

REUTERS CH BS2152