London (AFP) - The United States and former colonial power Britain on Thursday voiced alarm over the jailing of leaders of Hong Kong's democracy movement and urged China to allow free expression.

Four prominent leaders were jailed on Wednesday over their role in the 2014 Umbrella Movement, which paralysed Hong Kong for months and infuriated Beijing with its show of anger over the city's leadership and direction.

"The sentences handed to the 'Occupy' activists in Hong Kong are deeply disappointing," British Foreign Secretary Jeremy Hunt said in a statement on Twitter.

Britain said that the "One Country, Two Systems" concept -- enshrined in a Joint Declaration under which Britain handed the metropolis to China in 1997 -- guaranteed political freedom.

"It would be deeply concerning if the outcome for these individuals were to deter the people of Hong Kong from participating in peaceful protest in the future," a spokeswoman for British Prime Minister Theresa May said separately.

"Freedom of speech and freedom of assembly are both guaranteed by the Sino-British Joint Declaration and it is important that these... are fully respected," she said.

The United States also said it was "disappointed" and called on Hong Kong to respect "residents' rights of freedom of speech and peaceful assembly."

"Societies are best served when diverse political views are respected and can be freely expressed," State Department spokeswoman Morgan Ortagus said in a statement.

"Continued erosion of the 'One Country, Two Systems' framework puts at risk Hong Kong's long-established special status in international affairs," she said.

Hong Kong enjoys rights unseen in mainland China, but activists have warned of a steady erosion of freedoms.

The United States also voiced concern over Hong Kong's plans for an extradition treaty with mainland China, under which residents of the international financial hub could become entangled in the communist nation's opaque courts.