The UK and the US have condemned the 10-year sentence given to Hong Kong pro-democracy activist Joshua Wong under a controversial national security law.

Wong, 30, was sentenced on Thursday after pleading guilty to “foreign collusion” under the law. He is the only person to have been prosecuted twice under the legislation that critics say has been used to silence political opposition in the city.

A spokesperson for the UK Foreign Office said that Wong’s sentence was “a clear example of how the National Security Law is being used to target individuals for political expression and participation”.

The spokesperson added: “We call for an immediate end to all unjust prosecutions of individuals charged under the National Security Law and urge the authorities in China and Hong Kong to honour the rights and freedoms guaranteed to the people of Hong Kong under the Sino-British joint declaration (1984).”

Pro-democracy umbrella movement protesters on the streets of Hong Kong in 2014. Photograph: Alex Hofford/EPA

The US consulate in Hong Kong said in a statement that it “strongly condemns” the additional sentence and called for Wong’s immediate release. “Wong has been unjustly imprisoned since December 2020, demonstrating that Beijing will go to extraordinary lengths to silence those who advocate for fundamental freedoms in Hong Kong,” a consular spokesperson said.

Shortly after the US consulate strongly condemned the sentence on Thursday, China warned “relevant parties” to “stop making irresponsible remarks” about Hong Kong judicial cases.

Human rights groups and politicians from across the political spectrum also criticised the sentence.

Elaine Pearson, Asia director at Human Rights Watch, said: “The harsh sentence on baseless charges against Joshua Wong is cruel and unjust. Young people promoting social change are an asset for any vibrant society, but the Hong Kong government puts them behind bars.”

Sarah Brooks, deputy regional director for Amnesty International, said: “Today’s sentence is another grim reminder of how Hong Kong’s National Security Law has become a tool to punish dissent rather than to genuinely protect national security.”

She said the new conviction “cruelly – and perhaps deliberately – comes mere months before he would have been eligible for release under his previous conviction”.

Wong has already spent six years in detention and had been due for release in January 2027.

In the most recent Foreign Office report on Hong Kong, published last month, the UK’s foreign secretary, Ed Miliband, said that Wong’s case “reinforced concerns about the increasingly broad application of national security legislation”.

Wong’s supporters called on governments around the world to push for his release.

Nathan Law, a Hong Kong activist and friend of Wong’s who was named as a co-conspirator in the case, said: “It’s incredibly sad to see my friend Joshua Wong suffer another sentencing over his peaceful advocacy work for Hong Kong’s democracy.”

Law called the sentencing “a farce” which signified that China “is determined to quash any hopes for freedom and democracy on its soil”.

When former British colony Hong Kong was handed to China in 1997, it was promised 50 years of “one country two systems”, under which the city would keep some autonomy and freedoms.

The UK considers Beijing to be in violation of that international treaty, after the crackdown on protests in 2019 and 2020, and the sweeping legal reforms that followed.

Conservative MP Iain Duncan Smith said Wong had “stood up” for that treaty, which he said China had “subsequently trashed”.

He added: “Despite this, the UK government has done little to nothing about it. The more we give in when China breaks the rules, the worse it gets in the future.”

The Labour MP James Naish said: “Hongkongers in my constituency chose Britain for the freedoms Joshua is jailed for defending – values we should all support. This is not a lenient sentence and the government must show China that Britain will not tolerate it.”