G7 leaders meeting in France on Monday backed Hong Kong's autonomy as laid out in a 1984 agreement between Britain and China and called for calm in the protest-hit city.

"The G7 reaffirms the existence and the importance of the 1984 Sino-British agreement on Hong Kong and calls for avoiding violence," according to a joint statement issued in French at the end of a G7 summit in Biarritz, southwest France.

British Prime Minister Boris Johnson told reporters the leaders of Britain, Canada, France, Germany, Italy, Japan and the United States France had all voiced "deep concern" about the situation in Hong Kong.

"The G7 nations all want to support a stable and prosperous Hong Kong and we remain collectively committed to the one-country, two-systems framework," he said.

Hong Kong has been wracked by more than two months of protests over an attempt by its Beijing-backed government to pass an extradition bill which opponents saw as a huge dent in Hong Kong's autonomy.

They have since morphed into a call for greater democracy and police accountability, with youths at the forefront of demonstrations that have at times descended into clashes with security forces.

On Monday, police used water cannons to repel what they termed "extremely violent" demonstrators, following another weekend of clashes at pro-democracy rallies.

Twenty-one officers were injured during the clashes, Hong Kong police said, while dozens of protesters -- including a 12-year-old -- were arrested for unlawful assembly, possession of weapons and assaulting police.

Officials worldwide have urged Hong Kong authorities to show restraint to avoid serious violence or death.

China has accused former colonial power Britain, which has rebuked Beijing over its handling of the demonstrations, of meddling in the semi-autonomous Chinese territory which the British handed over to China in 1997.