Hong Kong protesters are expected to rally outside the British Consulate on Sunday demanding that the former colonial power ensures China honours its commitments to the city's freedoms.
The Chinese-ruled territory has been rocked by weeks of sometimes violent pro-democracy protests, with demonstrators angry about what they see as creeping interference by Beijing in their city's affairs despite a promise of autonomy.
The Sino-British Joint Declaration, signed in 1984, lays out Hong Kong's future after its return to China in 1997, a "one country, two systems" formula that ensures freedoms not enjoyed on the mainland.
"We ask UK to take immediate action on China not honouring the Sino-British Joint Declaration and acknowledge one country, two systems is not functioning," the rally organisers said.
The spark for the protests was planned legislation, now withdrawn, that would have allowed people to be sent to mainland China for trial, despite Hong Kong having its own much-respected independent judiciary.
The protests have since broadened into calls for universal suffrage.
China says it is committed to the "one country, two systems" arrangement, denies meddling and says the city is an internal Chinese issue. It has accused foreign powers, particularly the United States and Britain, of fomenting the unrest and told them to mind their own business.
Britain says it has a legal responsibility to ensure China abides by the 1984 declaration.
"The Joint Declaration is a legally binding treaty between the UK and China that remains as valid today as it was when it was signed and ratified over 30 years ago," a British Foreign Office spokeswoman said in June.
"As a co-signatory, the UK government will continue to defend our position."
But it was not immediately clear what Britain could do. It is pinning its hopes on closer trade and investment cooperation with China, which since 1997 has risen to become the world's second-largest economy, after it leaves the European Union as planned at the end of next month.
Meanwhile, Pro-democracy activist Joshua Wong said on Saturday he was seeking the support of US lawmakers for the demands of his fellow protesters who have led months of streets demonstrations, including a call for free elections.
Wong, who spoke to Reuters in New York ahead of a planned visit to Washington, led Hong Kong's pro-democracy "Umbrella Movement" in 2014. The latest protests, which began over a now-withdrawn extradition bill but grew into demands for greater democracy and independence from mainland China, are mostly leaderless.
"We hope ... for bipartisan support," Wong said of his trip to Washington, adding that US lawmakers should demand the inclusion of a human rights clause in ongoing US-China trade negotiations.
He said he also hoped to convince members of Congress to pass the Hong Kong Human Rights and Democracy Act, which would require an annual justification of the special treatment that for decades has been afforded to the city-state by Washington, including trade and business privileges.
The bill would also mean that officials in China and Hong Kong who undermined the city's autonomy could face sanctions. Democratic US Senator Chuck Schumer said earlier this month that it would be a priority for US Senate Democrats in their new session, which started on Monday.
The Civil Human Rights Front has also called for a mass rally in Victoria Park, just to the east of the central business district, but police have denied permission because of earlier clashes after huge gatherings.
The rally is expected to start at around noon (0400 GMT). Rain is forecast, but that hasn't stopped protests before.
Protesters are expected to turn up early in the afternoon anyway.
China is eager to quell the protests before the 70th anniversary on Oct. 1 of the founding of the People's Republic of China. The nation has not involved its military in efforts to quash the protests.
Wong said the Hong Kong people would keep fighting for their cause through the anniversary.
"We will continue our protest with our course on free elections," he said. "I see no reason for us to give up and it's time for the world to stand with Hong Kong."