UN chief Ban Ki-moon on Tuesday urged Chinese authorities and Hong Kong protesters to peacefully resolve their differences after riot police fired tear gas at pro-democracy protesters.

The UN secretary general "understands that this is a domestic matter, but urges all stakeholders to resolve any difference in a manner that is peaceful and safeguards democratic principles," said his spokesman.

Thousands of Hong Kong demonstrators rejected demands from the city's chief executive Leung Chun-ying to end rallies seen as the most intense civil unrest since the 1997 handover from British rule.

On Sunday, riot police fired clouds of tear gas at the protesters who are opposing a Beijing decision to vet candidates ahead of the election in 2017 of the city's next leader.

The United States on Monday called for restraint and British Deputy Prime Minister Nick Clegg on Tuesday said he will summon the Chinese ambassador to hear his "dismay and alarm" about the elections decision.

British deputy PM summons Chinese envoy over HK protests
London (AFP) Sept 30, 2014 - British Deputy Prime Minister Nick Clegg said on Tuesday he will summon China's ambassador to London over the protests in Hong Kong to express his "dismay and alarm" about the refusal to grant free elections.

"I'll be summoning the Chinese ambassador to express to the ambassador my dismay and alarm," he said, according to a statement from his office, with a spokesman saying only that the meeting had been requested for this week.

"The Chinese authorities in Beijing seem determined to refuse to give to the people of Hong Kong what they are perfectly entitled to expect, which is free, fair, open elections based on universal suffrage, as guaranteed by the joint declaration signed by the Chinese and British governments," he said.

Britain handed control of Hong Kong to China in 1997 under an agreement that enshrined the "one country, two systems" principle, which was meant to preserve Hong Kong's capitalist system and way of life for a period up to 2047.

"Universal suffrage means what it says on the tin. It means everybody can vote, and everybody can vote for the candidates they want. Not for candidates that have been screened and pre-selected by the authorities in Beijing," Clegg said.

"I really do think it's very important at this time... that we say very loud and clear that we are on their side," he said.

Clegg, the leader of the Liberal Democrats, the junior partner in Prime Minister David Cameron's Conservative-led government, said speaking to the ambassador "should help" as part of international pressure on China.

"It is not in China's interests to have increasingly large numbers of disenchanted people, citizens, in Hong Kong, who feel that they have been robbed of their right to exercise their democratic right," he said.

Clegg said he had been "hugely impressed" by the peaceful protests, adding: "I think people power is one of the great powerful forces in society."

Cameron earlier said he was "deeply concerned" about the escalating protests.

He told Sky News television: "When we reached the agreement with China there were details of that agreement about the importance of giving the Hong Kong people a democratic future."

The demonstrators have demanded full universal suffrage after Beijing last month said it would allow elections for the semi-autonomous city's next leader in 2017 but would vet the candidates -- a decision branded a "fake democracy".

Beijing on Tuesday called the street protests "illegal".