More than 600 people in Belarus have been charged with public disorder offences and five presidential candidates remained in prison today after a violent crackdown on the protests that followed the re-election of Alexander Lukashenko as president.

Tens of thousands of protesters flooded into central Minsk on Sunday night after the country's electoral commission reported that hardliner Lukashenko had won 80% of the vote. Police beat demonstrators with batons and loaded hundreds into prison buses known as avtozaki.

Opposition candidates, including the former deputy foreign minister Andrei Sannikov and the poet Vladimir Neklyayev - who was dragged from a hospital bed early yesterday after being hit in the face - are reportedly being held at a prison without being charged.

Alexei Gavrutikov, a representative of candidate Nikolai Statkevich, told news agencies Statkevich had been "kidnapped" outside the main post office building. "People in plain clothes, without producing any documents, dragged Statkevich out of a taxi, beat him and took him in an unknown direction," Gavrutikov said.

Two other candidates who had been detained were released yesterday evening.

The demonstrators were protesting that Lukashenko had crushed opposition and inflated his share of the vote in the former Soviet state - a claim backed by election monitors from the Organisation for Security and Co-operation in Europe (OSCE).

The observers reported yesterday that media coverage in the runup to the election had been hugely skewed in Lukashenko's favour and that the vote count was "flawed" and "bad or very bad in almost half of all observed polling stations".

The OSCE added that "election night was marred by detentions of most presidential candidates, and hundreds of activists, journalists and civil society representatives".

Sannikov's sister, Irina Bogdanova, said a lawyer had been allowed to see him today, but was obliged to sign a gagging document forcing him not to speak about the visit. "Andrei's leg is injured, and we're not sure if he is receiving treatment," she told the Guardian. "As far as we know, he has not been charged. It's horrendous."

About 600 people have been sentenced from five to 15 days in prison. Some protesters were released without charge, although it was unclear how many.

Among those set free was Natalya Kolyada, one of the leaders of the Belarus Free Theatre, which has gained support from prominent cultural figures in the west including Tom Stoppard. Kolyada's husband, Nikolai Khalazin, said the theatre's manager, Tema Zheleznyak, had been sentenced to 11 days in prison.

Lukashenko defended the crackdown in extraordinary remarks at a press conference, saying: "I did warn you that, if any mess started, we'd have enough resources [to put it down]," he said. "You guys took on the wrong person. I won't hide in a cellar. So let's put paid to it. There will no longer be any brainless democracy. We won't have the country torn to pieces."

The president said that when he was informed of Neklyayev being beaten, he asked the health minister, Vasily Zharkov, to take charge because "there will be such so much stench", to which Zharkov said, laughing, that "there is just a scratch on the back of his head and a bruised eye, there's no threat to his life at all".

Another candidate, Vitaly Rimashevksy, "was struck in the head and rushed to take shelter in a hospital", Lukashenko added. "And they wanted to be presidents! What kind of a president can you be if you shout to the whole world after being struck in the face? You must endure."