In 2010, the Belarussian opposition leader Andrei Sannikov took part in the country's presidential election. He was under no illusions he might "win".

Since taking power in 1994, the country's hardline president, Alexander Lukashenko, had maintained an iron grip on power. But Sannikov was unprepared for the regime's sudden, violent crackdown, the worst in 20 years.

On the evening of the vote, Sannikov and nearly 30,000 opposition supporters rallied in Minsk's freezing central square. His recollection of what happened next is hazy.

Riot police grabbed Sannikov, pushed him to the ground and then beat him savagely. "I lost consciousness," he says, speaking in his first newspaper interview since fleeing Belarus. "My wife and friend covered me with their bodies. They saved my life."

When he came round, Sannikov couldn't walk. A secret police officer had smashed his knees with a metal shield, he says – his leg was in agony. Friends helped him stagger into a journalist's car.

On the way to hospital police officers stopped his vehicle and dragged him out. They began beating him again. "I heard my wife screaming," he says. His arrest followed a pre-planned "invasion" of parliament by government provocateurs.

The police took him to the KGB's notorious Minsk prison, known by locals as the Americana (after the collapse of the Soviet Union, Russia rebranded its domestic spy agency as the FSB – Lukashenko kept the old name). Here, guards refused him medical treatment. He was locked in a cell, still wearing his candidate's ID badge. Friends, family and the international community feared the worst; it was weeks before news emerged he was alive.

Sannikov, a former diplomat, is Belarus's most high-profile opposition figure. He spent the next 16 months in jail. In May 2011, he received a five-year prison sentence for "inciting mass disorder". He and other inmates were kept in humiliating conditions, forced to strip naked for regular searches.

At one point the regime encouraged him to kill himself. It left him in an isolation cell with a razor blade and a piece of cord.

Sannikov was released in April. In August Lukashenko – Europe's last dictator in the words of former US secretary of state, Condoleezza Rice – suggested Sannikov would soon be rearrested. Reluctantly, he sought political asylum in Britain.

Sannikov's wife, Iryna Khalip, and their five-year-old son, Dania, remain in Minsk, effectively hostages of the Lukashenko regime.

Speaking from London, his new home, Sannikov says Belarus's reputation as Europe's most repressive state is deserved. "Lukashenko is a dictator. He openly calls himself that. Sometimes he tries to be coquettish and says he isn't, but he's admitted it several times. I think it's true," he says. Like other dictators, the president has a ruthless "animal instinct" for power, he adds.

In exile, Sannikov is divided from Khalip, a well-known journalist with the Russian investigative newspaper Novaya Gazeta. She too was jailed during the December 2010 demonstration; the authorities threatened to take her son into care. She then spent months under KGB house arrest. Two agents lived in her flat; she was forbidden from using the phone, emailing, or going near the window.

Last month, Lukashenko "promised" to free her during a meeting with the newspaper proprietor Evegeny Lebedev, who requested her release. Despite this, she is forbidden to leave Minsk, join her husband or work as a journalist.

Sannikov describes Belarus's system of government as a kolkhoz dictatorship (kolkhoz is the Russian word for collective farm; previously Lukashenko was a farm manager and KGB border guard). The clampdown that followed the presidential election marked the victory of the siloviki, hardliners led by Viktor, one of Lukashenko's sons.

"The system has no ideology. It denies national values such as history, culture and language. It's based only on the necessity to keep power," Sannikov says.

Aged 58, Sannikov grew up in Minsk. He studied English and joined the Soviet foreign service; after Belarus's independence in 1991, he served as a diplomat in the US and Switzerland. In 1995 he became Belarus's deputy foreign minister. He resigned in protest at Lukashenko's policies and co-founded a pro-democracy group, Charter 1997. He was one of several presidential candidates arrested by Lukashenko after the 2010 poll; officially he came second.

During nearly two decades in power Lukashenko has performed an east-west foreign policy balancing act, playing off Brussels against Moscow. After the 2010 crackdown the EU reimposed travel bans on Lukashenko and 150 officials. But according to Sannikov, the democratic world is too "complacent" about Belarus. The post-Soviet state isn't only domestically repressive, he says, but sells weapons to rogue governments and entities around the world.

"Belarus poses a threat to international security. Dictators are very good at consolidating themselves. They form a club," he argues.

This club, Sannikov suggests, is getting larger. The opposition leader says he is "pessimistic" about Belarus's neighbour Russia, where President Vladimir Putin has launched his own crackdown on civil society since returning to the Kremlin in May.

"Russia is heading the way of Belarus," he notes. Ukraine, and other post-Soviet states such as Georgia, are going in the same backward direction, he believes.

Sannikov heard about last year's uprisings in the Middle East from his prison cell. He recalls: "When I heard about the Arab Spring I had an almost physical feeling." "We had prepared everything in Belarus. But someone above decided to strike in another place. It wasn't fair on Belarus," he says wryly. He rejects the idea that Lukashenko, self-styled father of the nation, enjoys mass popular support. Belarus's severe economic crisis means that living standards are falling, despite bail-outs from Russia.

"The prevailing majority in Belarus would welcome changes," he says.

For the moment, Lukashenko survives, through a mixture of terror, the KGB, and old-fashioned Soviet populism. While in jail, the head of Belarus's KGB, Vadim Zaitsev, interrogated him. "It would be funny were it not so tragic. They don't change, these people. They lie to you. They use false information, false accusations," Sannikov said, recalling how Zaitsev accused him of consorting with foreign "spies" and being a puppet of the west.

Belarus is a small, historically luckless country of 9.5 million. Outsiders often see it as a sort of Soviet Union theme park, synonymous with gloomy, totalitarian rule. Living conditions have collapsed, with massive inflation and a currency in freefall. Despite this, Sannikov says, Belarus has a lot to offer. It has a thriving cultural scene, with first-class ballet and music, as well as lakes, forests and great natural beauty. He is convinced that given a chance a post-Lukashenko Belarus could rejoin the western mainstream, and become a thriving European state.

Sannikov says he is convinced Lukashenko's regime is "doomed".

"I see no future for it," he says. He won't predict when it will fall, but adds: "It's only a matter of time."