The UK is on the side of Belarusian opposition leaders trying to bring down the tyrannical regime led by Alexander Lukashenko, Boris Johnson has said.

The prime minister gave his full support to Sviatlana Tsikhanouskaya, who is at the forefront of efforts to restore democracy in the face of a crackdown on civil society in the east European country.

Hosting Tsikhanouskaya in Downing Street hours after the head of a group that helps Belarusians fleeing persecution was found dead in a park in Kyiv, the Ukraine capital, Johnson said the UK backed Tsikhanouskaya’s struggle against severe human rights violations and the persecution of pro-democracy activists.

“We are very much on your side, very much in support of what you are doing. We are committed to supporting human rights and civil society in Belarus,” he told her on Tuesday.

Tsikhanouskaya underlined the power of Johnson’s declaration of support, saying it was “very important to understand that one of the most powerful countries in the world are supporting Belarus”.

The prime minister replied: “We strongly support you, strongly support Belarus, the Belarusian people.”

He also highlighted the speed at which the UK announced sanctions against Belarus, after the “hijacking” of a plane carrying another prominent anti-Lukashenko organiser, Raman Pratasevich, who was taken into custody by the country’s authorities at the end of May after a Ryanair flight was forced to land in Belarus.

No 10 said in a readout of a private meeting the pair held later that Johnson told Tsikhanouskaya both Britons and Belarusians “share fundamental values such as a belief in democracy, human rights and rule of law”, and that the UK “stands in solidarity of the people of Belarus and will continue to take action to support them”.

The significance of Johnson’s support for Belarusian opposition leaders was intensified given the death of Vitaly Shishov, the head of Belarusian House in Ukraine. Shishov was reported missing by his partner on Monday after he did not return from a run and could not be reached on his mobile phone. Police said in a statement he had been found hanged in a park not far from where he lived, and that they had opened a murder investigation.

It also followed the recent effort by the Belarus Olympic athlete Krystsina Tsimanouskaya to seek asylum. She received a humanitarian visa from Poland after she was threatened with being repatriated to Minsk over her criticism of Belarus Olympic team officials.

Outside No 10, Tsikhanouskaya told reporters it was too early to comment on Shishov’s death, but said she would be undeterred in fighting for free and fair elections in Belarus.

“I understand you know, I can disappear at any moment,” she said. “I understand this, but I should do what I am doing. I can’t stop, because I feel responsibility for the future of my country, the same as all those Belarusians felt fighting the government, feel their responsibility.

“But I know that even if I disappear one day, this movement will continue without me.”

Further announcements are expected from the UK government over the summer as it raises pressure on Belarus to curb the recent raids of rights groups, media organisations, charity groups and other non-governmental organisations. Lukashenko is seeking to stamp out even apolitical efforts by Belarusians to self-organise.

The Viasna human rights centre recently said the raids and arbitrary arrests were “just another instance of the crackdown against human rights defenders, civil society organisations and independent media that has been going on since the widely disputed presidential election in August 2020, when thousands of Belarusians took to the streets in mostly peaceful protests”.