HONG KONG, Aug 15 (Reuters) - Hong Kong braced on Thursday for more mass demonstrations through the weekend, with the weeks-long crisis escalating after pro-democracy protests forced the cancellation of nearly 1,000 flights this week and world leaders urging calm.

China reiterated on Wednesday that Hong Kongs protest resembled terrorism and more street clashes followed ugly and chaotic scenes at the airport two days ago, when protesters set upon two men they suspected of being government sympathizers.

Police and protesters faced off again on the streets of the financial hub overnight, with riot officers quickly firing teargas as their response to demonstrators hardens.

Ten weeks of increasingly violent confrontations between police and protesters have plunged Hong Kong into its worst crisis since it reverted from British to Chinese rule in 1997.

The protests represent one of the biggest populist challenges for Chinese President Xi Jinping since he came to power in 2012 and show no immediate signs of abating.

U.S. President Donald Trump tied a trade agreement withChina to a humane resolution of the protests that have disrupted the city for the past 10 weeks, even suggesting that he was willing to meet Xi to discuss the crisis.

I have ZERO doubt that if President Xi (Jinping) wants to quickly and humanely solve the Hong Kong problem, he can do it.Personal meeting? Trump said on Twitter.

The U.S. State Department said earlier it was deeply concerned about reports that Chinese police forces were gathering near the border with Hong Kong and urged the citys government to respect freedom of speech.

It also issued a travel advisory urging citizens to exercise caution when visiting Hong Kong. China has frequently warned against what it regards as outside interference in an internal issue.

French Foreign Minister Jean-Yves Le Drian called on HongKong authorities on Wednesday to renew talks with protesters to find a peaceful solution, while Canadian Prime Minister JustinTrudeau urged China to handle the protests with tact.

AIRPORT REOPENS

Hong Kongs Airport Authority said normal flight operations would resume on Thursday but heightened security would remain at the citys international airport. It said on Wednesday an application for protests to be held in the terminal must be made in advance with a Letter of No Objection from police.

Protesters have expressed remorse after a peaceful sit-in turned violent at one of the worlds busiest airports earlier this week.

It was not yet clear whether the violent clashes had eroded the broad support the movement has so far attracted in HongKong. The protests have also hit the citys faltering economy.

Business and citizens groups posted full page advertisements in major newspapers to support the government and denounce the violence.

The Chinese Securities Association of Hong Kong said the citys international reputation would be seriously damaged if the violence and unrest were not stopped as soon as possible.

The head of Macau casino operator Galaxy Entertainment, Lui Che-woo, urged talks to rebuild a harmonious HongKong. The protests have affected the neighboring Chinese territory of Macau, with some visitors avoiding the worlds biggest gambling hub amid transport disruptions and safety concerns.

Several protests were planned across different districts ofHong Kong from Thursday, including a teachers rally, and one organized by animal lovers upset that their pets were being tear-gassed. The Civil Human Rights Front, which organized million-strong marches in June, set another protest for Sunday.

Protesters are still pushing for authorities to listen to their five requests, which include the complete withdrawal of a now-suspended extradition bill that would have allowed criminal suspects to be sent for trial in mainland Chinese courts.

The protests grew out of opposition to the extradition bill into wider concerns about the erosion of freedoms guaranteed under the one country, two systems formula put in place after the return to Chinese rule in 1997.

Their other demands include a halt to descriptions of the protests as rioting, the dropping of charges against those arrested, an independent inquiry, and the resumption of political reform.

(Additional reporting by Donny Kwok and Twinnie Siu in HongKong, David Brunnstrom and Jonathan Landay in WASHINGTON,Mathieu Rosemain in PARIS, and David Ljunggren in OTTAWA;Writing by Farah Master; Editing by Paul Tait)