The move follows the approval on Wednesday by China's National People's Congress Standing Committee of a resolution giving Hong Kong authorities the power to bypass local courts and summarily remove politicians seen as a threat to security.

Four Hong Kong lawmakers who support the territory's pro-democracy movement were quickly unseated, prompting opposition lawmakers to resign en masse as a show of solidarity.

The resolution effectively gives Beijing veto power over who sits in Hong Kong's Legislative Council, or Legco, and the mass resignation of the opposition leaves the assembly with no dissenting voices.

London on Thursday accused China of breaching the agreement that laid the groundwork for the handover of Hong Kong nearly a quarter century ago. The Sino-British Joint Declaration, signed in 1984, spelled out the "one country, two systems" philosophy meant to afford the former British colony a large measure of autonomy.

The U.K.'s minister for Asia, Nigel Adams, told Parliament that London was considering sanctions against individuals in China. "It's not entirely appropriate to speculate who may be designated under the sanctions regime in the future as that could reduce the impact, but we are carefully considering further designations under the scheme."

The EU on Thursday called Beijing's move a "severe blow" to Hong Kong's autonomy and called for the resolution to be immediately reversed.

"This latest arbitrary decision from Beijing further significantly undermines Hong Kong 's autonomy under the 'One Country, Two Systems' principle," the EU's 27 governments said in a statement. "These latest steps constitute a further severe blow to political pluralism and freedom of opinion in Hong Kong," they said.

U.S. national security adviser Robert O'Brien on Wednesday also criticized the move, saying in a statement: " 'One Country, Two Systems' is now merely a fig leaf covering for the CCP's expanding one party dictatorship in Hong Kong," referring to China's Communist Party.

On Monday, the Trump administration announced sanctions on an additional four Chinese officials in response to the national security law.

In July, the Trump administration signed legislation and an executive order sanctioning Beijing for its national security law.

"Hong Kong will now be treated the same as mainland China," Trump said at the time. "No special privileges, no special economic treatment and no export of sensitive technologies."

The 15 remaining opposition lawmakers walked out of the chamber on Thursday. In an act of defiance, one of them unfurled a banner saying of the city's Beijing-appointed leader, Carrie Lam, that she will "stink for 10,000 years." Lam has defended the ouster of the four legislators as lawful.

Beijing's liaison office called the mass walkout a "farce" and said it demonstrated the lawmakers' "stubborn resistance" to China's authority.

"Opposition lawmakers have used their public post as a tool of political manipulation," a spokesperson said for China's Hong Kong and Macau Affairs Office. "If these lawmakers hope to make use of their resignation to provoke radical opposition and beg for foreign interference, they have miscalculated."

Democratic Party chairman Wu Chi-wai said the resignations won't officially take effect until Dec. 1. "This is for us to handle the dismissal of our staff members and assistants," he said.

Last year, major protests in Hong Kong erupted over a proposed extradition bill that would have allowed some people accused of crimes in the territory to be transferred to mainland China to face courts there. The bill was later withdrawn, but Beijing later approved a sweeping national security law that provides for harsh penalties for supporting secession, subversion of state power, terrorism or collusion with foreign entities.