HONG KONG — Demonstrators smashed their way into Hong Kong’s legislative building Monday, storming the main chamber, spraying graffiti on the walls and hoisting the flag that the city used before it was governed by China.

The protesters defaced Hong Kong’s emblem, tore down portraits of legislative leaders and wrote messages on the chamber’s high wooden wall calling for the resignation of Carrie Lam as as Hong Kong’s chief executive and leader.

Then shortly after 12 a.m. Tuesday (noon ET, Monday) police in riot gear began throwing tear gas at protesters outside.

By this point most of the protesters had left the legislative chambers, with people chanting inside “leave together.” Only four were remaining inside as the clock hit midnight.

Earlier, police had tried to hold back the protesters by using riot shields and pepper spray, and the secretariat of the Legislative Council issued a “red alert” for all staff inside to immediately evacuate the building.

Police said protesters “violently attacked and forced” their way in “illegally.”

“The police severely condemned the violent attack,” according to a police statement. “The police will conduct sweeping in a short period of time and will take reasonable force.”

The activists are calling for the total withdrawal of a proposed law that would allow suspected criminals to be extradited from Hong Kong to the Chinese mainland.

Outside, some 550,000 people were flooding the city’s streets for the annual July 1 march, marking the day in 1997 when this former British colony was handed back to China.

The march is usually a general demand for democracy and human rights. But this year’s procession carries much sharper resonance, coming after around a month of mass protests against the proposed extradition law.

The city was supposed to retain its own political and economic system after leaving U.K. rule. Critics say the law is part of attempts by Beijing to erode those freedoms, edging Hong Kong toward China’s political and legal system, which rights groups say is rife with abuses and is used to silence political opponents and dissent.

British Foreign Secretary Jeremy Hunt on Monday walked a fine line, declaring that Hong Kong’s former rulers are “unwavering” in their support of democracy while also saying “no violence is acceptable.”

He said Hong Kong citizens must “preserve right to peaceful protest exercised within the law, as hundreds of thousands of brave people showed today.”

Hong Kong backers of the extradition bill said it’s necessary to stop the semi-autonomous region from becoming a haven for fugitives.

The government indefinitely suspended it after the protests and Lam apologized. But this has not satisfied the protesters, who want the bill to be scrapped for good and for Lam to resign.

One student, Rick, 23, was one of those moving barriers outside the government building. He said he did not smash anything himself but understood why others were doing so.

“People are fed up, the anger is building up and they need to vent it out,” he said, declining to give his last name for fear of prosecution by authorities. “I think this is what triggered what is happening now.”

Another, Lindsey, 21, also declining to give her full name, said, “It’s tiring, but this is the only thing we can do.” She added, “If we don’t come out, we never know if we will have the chance again in the future.”

Earlier, protesters clad in masks and hard hats tried to advance down closed streets toward a venue where a flag-raising ceremony was taking place marking Hong Kong’s handover.

They used a large metal cart and other objects to smash the glass walls of the legislative building.

Shielded by dozens of others wielding umbrellas — a symbol for Hong Kong protests in 2014 — they later ripped down at least one of these panes and dozens flooded inside. Elsewhere, protesters tore off the metal bars of a gate leading into the building.

The larger demonstration outside capped more than a month of mass demonstrations in Hong Kong, the largest of which saw some as many as 2 million of the city’s 7 million residents flood the streets.

Veta Chan reported from Hong Kong, Alexander Smith reported from London and David K. Li reported from New York.

David K. Li contributed.