The Hong Kong government said more than 50 people had been detained on suspicion of seeking to overthrow or interfere with the pro-China administration.
The former politicians and pro-democracy activists were accused of violating the new national security law by participating in unofficial elections for the legislature last year.
The mass arrests were the largest move against Hong Kong’s democracy movement since the law was imposed by Beijing last June to quell dissent in the semi-autonomous territory.
“The operation today targets the active elements who are suspected to be involved in the crime of overthrowing, or interfering (and) seriously destroy the Hong Kong government’s legal execution of duties,” said John Lee, Hong Kong’s security minister.
He added that those arrested were suspected of trying to paralyse the government by planning to gain a majority of seats to force the region’s chief executive Carrie Lam to resign and the government to stop functioning.
But Mr Raab said: “The mass arrest of politicians and activists in Hong Kong is a grievous attack on Hong Kong’s rights and freedoms as protected under the joint declaration.
“These arrests demonstrate that the Hong Kong and Chinese authorities deliberately misled the world about the true purpose of the national security law, which is being used to crush dissent and opposing political views.
“The UK will not turn our backs on the people of Hong Kong and will continue to offer BNOs (British Nationals (Overseas)) the right to live and work in the UK.”
In a video released by a former lawmaker, Lam Cheuk-ting, police turned up at his house and told him he was “suspected of violating the national security law, subverting state power”. Police told those recording the video to stop or risk arrest.
The election that would have followed the unofficial primaries was postponed by a year by Mrs Lam, who cited the public health risks during the coronavirus pandemic.
Resignations and disqualifications of pro-democracy lawmakers have left the legislature largely a pro-Beijing body.
All of the pro-democracy candidates in the unofficial primaries last July were arrested, according to tallies by the South China Morning Post, online platform Now News and political groups.
Mr Lee also pointed to a plan allegedly formulated among those arrested. “The plot is to create such mutual destruction that if successful… will result in serious damage to society as a whole,” the security minister said, adding: “That is why police action today is necessary.”
Following the handover of Hong Kong to China by the British in 1997, the semi-autonomous Chinese city has operated on a “one country, two systems” framework that affords it freedoms not found on the mainland.