A Belarusian presidential candidate was dragged from his hospital bed and arrested today, just hours after police beat him and other protesters who rallied against an allegedly rigged election victory for the hardline leader, Alexander Lukashenko.

Men thought to be security agents carried candidate Vladimir Neklyayev, a 64-year-old poet, out of a ward where he was being treated for blows to the head at about 2am. His wife, Olga, told reporters: "We were in a hospital room when plainclothes men burst in, wrapped him up in a blanket and dragged out of the room in an unknown direction."

Six more of the nine opposition candidates in the election, including former deputy foreign minister Andrei Sannikov, were reportedly under arrest this morning as Lukashenko's regime moved swiftly to crush dissent.

Up to 400 activists were also detained, according to the Vyasna human rights group, which itself was stormed at 3.15am by members of the KGB, as Belarus's security agency is still called.

Tens of thousands of protesters converged on central Minsk last night, as exit polls indicated that Lukashenko - a Soviet-style dictator - would be awarded four-fifths of the vote. The crowds were dispersed by baton-wielding riot police, leaving several protesters bloodied.

Sannikov's sister, Irina Bogdanova, a doctor who lives in Britain, told the Guardian she had spoken to a friend of her brother, who said he saw police attacking the candidate. "Andrei was beaten up once and his friends helped him to a car," she said. "Then the police wrenched open the door, beat him again and dragged him off."

Sannikov's wife, Irina Khalip, a journalist, was taken away in a separate vehicle and had her head bashed against the door frame, said Bogdanova. "The special forces were kicking people in the head when they had fallen to the ground," she added. "It was absolutely barbaric."

Sannikov managed to text an acquaintance, saying: "I'm being taken to the KGB." Later in the day, secret police used his wife's key to enter their apartment, where Khalip's parents were looking after the couple's three-year-old son. "They were petrified," said Bogdanova.

Activists who attended the protests claimed provocateurs in the crowd broke windows in the main government building in order to justify a charge by riot police.

An international monitoring group from the Organisation for Security and Cooperation in Europe (OSCE) in Minsk today said the election was neither free nor democratic. "Election night was marred by the detention of most presidential candidates and hundreds of activists, journalists and civil society representatives," the organisation said. Belarus's election commission announced Lukashenko as the winner this morning with 80% of the vote.

The US embassy in Minsk condemned the violence and the EU called for detainees to be released immediately.

Activists who avoided arrest tried desperately to locate their detained comrades. Several opposition websites were blocked or intermittently online today including Charter97.org and Zapraudu.info, the site of Neklyayev's Speak the Truth! movement. Andrej Dynko, chief editor of the independent Nasha Niva newspaper, said: "Seven of the presidential candidates are unreachable by telephone and one of them, Grigory Kostusev, managed to send a text message saying he was delivered to Amerikanka, the KGB prison in Minsk. We think they are all there." Dynko said several hundred activists were being held at police stations and a pre-trial detention centre: "Preventative arrests began even before the protests. Neklyayev was attacked an hour before they started and beaten almost to death. It was an intimidation."

He added: "Lukashenko tried to make cosmetic changes and pretend he was liberalising Belarus - but he slipped back into his old, repressive ways."

The 56-year-old former collective farm manager has ruled Belarus with an iron grip since 1994 and his two re-elections since were judged unfair by the OSCE. About 10,000 people protested against his regaining the presidency in 2006, but police destroyed the opposition tent camp set up in Minsk's October Square.

Lukashenko has relied on political support from Moscow, but the relationship was shaken this year by disputes over Minsk's debts for Siberian gas and a Russian ban on dairy products from Belarus. Lukashenko also irked the Kremlin by attempting to make closer ties with the EU. Russian state TV launched a smear campaign against Lukashenko, with a series of documentaries called The God-Daddy, a play on the leader's nickname, Batka (Daddy) and Moscow indicated it was ready to support opposition candidates in the election.

However, the relationship appeared to be mended last week when Russia agreed to drop duties on oil exports to Belarus and keep natural gas prices low. The Russian-led CIS observer mission approved today's result and Russia's president, Dmitry Medvedev, said the elections were an "internal affair for Belarus". He added: "Hopefully, after the election, Belarus will continue to develop as a modern state based on democracy."

Lukashenko claimed this afternoon that he was saddened at having to send in riot police, but praised their tactics. "It took seven-and-a-half minutes to protect our country and its future," he said.

Dynko predicted there would be no more large-scale protests.

"Everyone who could organise them is in prison," he said.