The British government and MPs are to be the focus of a digital advertising campaign in which Hong Kong pro-democracy activists will plead for the UK to take action to protect rights in the former colony.

A group of activists have paid for adverts that appeared on Wednesday in British media outlets calling on the UK government to impose sanctions on those responsible for what is described as the suppression of human rights and freedoms in Hong Kong.

They also call on Britain to include provisions on human rights, civil liberties and democratisation in any post-Brexit agreements with Hong Kong and China.

The adverts accompany a digital campaign featuring images of Margaret Thatcher in Beijing for the signing of the 1984 Sino-British joint declaration that guaranteed freedom of speech, freedom of the press and other rights in Hong Kong.

“In 1984, Britain pledged to protect Hong Kong’s freedom. Britain must keep its promises,” say the adverts, which have been taken out by a grouping that is remaining anonymous in the face of fears of retribution by the Chinese government. The campaign has been by funded by donations, with £300,000 raised in less than 24 hours.

Members of the public are being urged to sign a petition. The campaign, launched by what is described as “a group of concerned Hongkongers around the world”, accuses China of consistently breaching the terms of the 1984 agreement. The latest breach, it says, came in the form of the extradition bill that has caused Hong Kong’s largest political crisis in decades.

Protests that have principally targeted the bill, which would allow suspects to be extradited from Hong Kong to mainland China, have taken on new demands, including an investigation into police use of rubber bullets, teargas and physical violence against protesters.

Protesters this week pledged to stand up to thugs who attacked demonstrators at the weekend as public anger grows towards the government and police.

Demonstrators have filed for a permit to hold a rally on Saturday in Yuen Long, the district on the outskirts of Hong Kong where dozens of masked men chased and beat commuters and protesters with wooden poles and metal rods, leaving at least 45 people in hospital. Police arrived after the assailants left.

A Foreign Office minister told the House of Commons on Monday that Britain remained fully committed to upholding Hong Kong’s high degree of autonomy, rights and freedoms under the “one country, two systems” principle.

Andrew Murrison said Britain would be keeping a “close eye” on the police investigation in Hong Kong into the attack in Yuen Long.

Dr Victor Gao, a former high-ranking Chinese diplomat, told the BBC on Tuesday that allegations that China had a hand in the attacks were groundless, describing allegations as “fake news”.

“At this particular moment let’s give due process more time. However, when rioting on such a large scale happened in Hong Kong no one should indulge in the fantasy that the people on the other side of the equation will wait until they are engulfed in instability,” he said.

“Villagers in Yuen Long … want to make sure that no rioters come to their village and I think let’s wait for the investigation.”