Protesters try to storm government HQ in Belarus
Earlier the police broke up a smaller rally. Opposition candidate Vladimir Neklyaev was seriously injured.
Incumbent Alexander Lukashenko is widely expected to win the poll, which the opposition claims has been rigged.
Exit polls suggest he is far ahead of his nearest rival in Sunday’s vote, having secured more than 70% of the vote.
Under the authoritarian leader now seeking his fourth term, the former Soviet republic has never held a poll seen as fair by international monitors.
However, the election campaign itself was much freer than in the past, correspondents say.
Injured opposition candidate Vladimir Neklyaev. Photo: 19 December 2010 Opposition candidate Vladimir Neklyaev was injured in the clash
‘Down with Gulag!’
Mr Neklyaev’s campaign activists told the BBC that he was badly beaten and taken to hospital with head injuries.
This happened as police violently broke up the rally of some 200 supporters of the opposition candidate after Sunday’s poll.