Fears are growing for two presidential candidates beaten by riot police and imprisoned in a brutal crackdown after disputed elections in Belarus.

Lawyers have been denied access to Vladimir Neklyayev, a 64-year-old poet, who was knocked unconscious during demonstrations on Sunday and dragged from his hospital bed to a KGB prison a few hours later.

"Neklyayev was badly beaten," said Tatyana Revyako of Viasna, a human rights group which is co-ordinating legal help to arrested demonstrators. "The fact we are not allowed to see him is extremely alarming and suspicious. He could be dead for all we know."

Andrei Sannikov, 56, who officially came second in the vote with 2.4%, was also beaten as he was arrested on Sunday evening, according to witnesses.

His lawyer, Pavel Sapelko, who visited him at the KGB's isolation unit, known as Amerikanka, on Monday, said he "could hardly walk" and looked "a terrible sight". "His leg is injured – either badly bruised or dislocated – and he is not receiving proper medical attention," Sapelko said. "He was put in a cell designated for three people but there are four in there, and he is forced to sleep on a sheet of wood on the floor."

David Lidington, the Europe minister, has passed on Britain's concerns about the welfare of the opposition politicians to the Belarusian ambassador to London, Aleksandr Mikhnevich. Lidington demanded that Neklyayev be given urgent medical attention and access to lawyers.

One British source said: "David asked the ambassador to tell his government that Britain is demanding the immediate release of everyone detained on political grounds. We want the whereabouts of Mr Neklyayev to be known and for him to have medical attention and access to legal advice. What we are seeing happening is inevitably going to have consequences for Belarus's relations with the EU."

Britain understands that Guido Westerwelle, the German foreign minister, and Radek Sikorski, the Polish foreign minister, are particularly angry with the president, Alexander Lukashenko. The two foreign ministers recently flew to Minsk to offer Lukashenko to offer a €3bn EU package of grants if he held reasonably free elections. "Westerwelle and Sikorski feel really angry and betrayed," one British source said. "Nobody is talking about the EU package now."

Mikhnevich was called to the foreign office to see Lidington on Wednesday. This is one step down from a formal summons. The ambassador responded to Lidington's complaints by reading out a pre-prepared defence of his government's position.

Belarus – often referred to as "the last dictatorship in the heart of Europe" – has been run for 16 years by Lukashenko, the 56-year-old strongman who has routinely crushed political dissent.

Tens of thousands of opposition supporters flooded into central Minsk on Sunday after election results showed Lukashenko winning 80% of the vote. A central square was left spattered with blood after baton-wielding police dispersed crowds.

On Monday and Tuesday more than 600 protesters were fined or sentenced to 10 to 15 days in prison and at least 19 people – including five presidential candidates – are in custody facing up to 15 years in jail on charges of "organising mass disorder".

Hillary Clinton, the US secretary of state, and Lady Ashton, the EU high representative, issued a joint statement today, saying they would review ties with Minsk and calling for the release of detainees. "We strongly condemn all violence, especially the disproportionate use of force against presidential candidates, political activists, representatives of civil society and journalists," they added.

Prosecutors have yet to charge any of the detained leaders, but Anatoly Kuleshov, the interior minister, said earlier this week: "Everyone will get what they deserve, according to their role, their actions. No one will go unpunished." He said the detainees felt "absolutely healthy and normal".

Lukashenko has branded the protesters "bandits" and praised police for their efficiency.

Opposition activists claim provocateurs in the crowds broke windows and doors in a central government building to justify the crackdown. A video circulating on the internet appears to show police loading protesters into a van, but releasing a man they recognise while shouting to each other: "Look out for ours."

Sannikov's sister, Irina, said: "This is not like the student protests in London when there's a confrontation, a few arrests and then people are released. My brother – an innocent, peaceful man – could stay in prison for 15 years. The world needs to take notice."

Mikhail Gorbachev, the former Soviet leader, wrote a letter to Lukashenko today asking him to free Sannikov's wife, Irina Khalip, saying she needed to care for her three-year-old son.

Khalip, a journalist who works for Novaya Gazeta, the newspaper owned by Gorbachev and the media tycoon Alexander Lebedev, was arrested as she gave a radio interview from the scene of the protest on Sunday.

Dmitry Medvedev, Russia's president, has made no comment on the events in Minsk, saying only they were "the internal affair of Belarus".

However, Russian state television aired a critical report of the crackdown last night, saying it was reminiscent of Joseph Stalin's repressions in the 1930s.

About eight Russian citizens are reported to be among those being held in prison. Moscow has urged Minsk to release them.