Joshua Wong, one of the leaders of the pro-democracy "Umbrella" movement five years ago, was re-arrested at Hong Kong's airport on Sunday for breaching bail conditions, he said.

He had been charged with inciting and participating in an unauthorized assembly outside police headquarters on June 21 and released on bail.

"Preliminary legal advice suggested that the court had acknowledged and approved my trips to Germany and the U.S. when it granted bail on Aug. 30," he said in a statement.

"Therefore, it is believed that there are some mistakes have been made on the bail certificate."

He was leaving for Germany and the U.S. to give speeches and interviews at the time of his arrest. Last week, he had travelled to Taiwan to speak to politicians about Hong Kong's pro-democracy movement.

In a statement released Sunday, pro-democracy activist Joshua Wong said his arrest is an example of political prosecution. (Tyrone Siu/Reuters)

He said he thought he would be freed on Monday.

1/ Joshua Wong delivered the following message through his legal representative: <br><br>I was arrested by police for “breaching bail conditions” this morning at the Airport’s customs and I am detained in custody now. —@joshuawongcf

News of his arrest came as demonstrators in Hong Kong marched to the U.S. consulate on Sunday, urging U.S. President Donald Trump to "liberate" them as they press for more democratic freedom in the semi-autonomous Chinese territory.

Thousands of people converged at a park in central Hong Kong, chanting "Resist Beijing, Liberate Hong Kong." Many of them, clad in black shirts and wearing masks, waved American flags and carried posters that read "President Trump, please liberate Hong Kong." Riot police stood watch as they began their march to the nearby U.S. consulate.

Thousands of Hong Kong protestors stream past US consulate pushing for more vocal American support in fight against China. Urging US to pass act that would punish Beijing if it breaks promises to allow freedoms in HK <a href="https://twitter.com/hashtag/CBC?src=hash&ref_src=twsrc%5Etfw">#CBC</a> <a href="https://t.co/H3P4jkWbv2">pic.twitter.com/H3P4jkWbv2</a> —@sasapetricic

"Hong Kong is at the forefront of the battle against the totalitarian regime of China," said Panzer Chan, one of the organizers of Sunday's march. "Please support us in our fight."

Hong Kong police later fired tear gas to disperse protesters, who dispersed to nearby Admiralty, the bar district of Wan Chai and on to the upmarket Causeway Bay shopping district.

Activists also set up barricades, smashed windows, started street fires and vandalized the MTR subway station in Central district. The area was awash in graffiti, broken glass and bricks torn up from pathways. Protesters set fires from cardboard boxes, building barricades with metal fencing.

Hong Kong police going block to block, randomly (it seems) firing tear gas at protestors, chasing them in circles through main HK commercial district, Causeway Bay (and guarding Victoria’s Secret) <a href="https://twitter.com/hashtag/CBC?src=hash&ref_src=twsrc%5Etfw">#CBC</a> <a href="https://t.co/ttRhl6GlKU">pic.twitter.com/ttRhl6GlKU</a> —@sasapetricic

"We can't leave because there are riot police," said protester Oscar, 20, in Causeway Bay.

"They fired tear gas from the station. We are heading to North Point."

North Point is east of Causeway Bay.

Hong Kong has been rocked by a summer of unrest kicked off by a proposed law that would have allowed criminal suspects to be sent to mainland China for trial. Many saw the extradition bill as a glaring example of the Chinese territory's eroding autonomy since the former British colony was returned to China in 1997.

Hong Kong's government promised last week to withdraw the bill — an early demand of protesters — but that has failed to appease the demonstrators, who have widened their demands to include other issues, such as greater democracy.

The unrest has become the biggest challenge to Beijing's rule since Hong Kong's return from Britain. Beijing and the entirely state-controlled media have portrayed the protests as an effort by criminals to split the territory from China, backed by hostile foreigners.

Protesters on Sunday urged Washington to pass a bill, known as the Hong Kong Democratic and Human Rights Act, to support their cause. The bill proposes sanctions against Hong Kong and Chinese officials found to suppress democracy and human rights in the city, and could also affect Hong Kong's preferential trade status with the U.S.

The U.S. State Department in a travel advisory Friday said Beijing has undertaken a propaganda campaign "falsely accusing the United States of fomenting unrest in Hong Kong." It said U.S. citizens and embassy staff have been the target of the propaganda and urged them to exercise increased caution.

Protesters called on U.S. President Donald Trump to 'liberate Hong Kong' during a march on Sunday. (Anushree Fadnavis/Reuters)

Some U.S. lawmakers have spoken out strongly in support of the Hong Kong protesters and voiced concern about the potential for a brutal crackdown by China.

U.S. President Donald Trump, however, has indicated the U.S. would stay out of a matter he considers between Hong Kong and China. He has said he believes the U.S. trade war with China is making Beijing tread carefully.

Sunday's rally followed overnight violent clashes between protesters and police at several metro stations.

Protesters set fire to debris near a metro station that had been shuttered in the crowded Mongkok area but retreated after riot police chased them using pepper spray.

Violent clashes separately took place at a station in Sha Tin new town, where protesters chased police officers into the control room before riot police arrived. Several people were injured and detained.