United Kingdom security officials said Britain is now "at war" following Saturday night's terror attack on London Bridge, and said intelligence gathering on Islamic extremists must be improved.

All main parties resumed campaigning for Thursday's general election on Monday, and many commentators said it was unlikely the attack would affect the result, as many people had made up their minds already.

"Make no mistake, we are now at war, and the way we handle intelligence has to change," a senior government official told China Daily. "Britain now faces two kinds of attack by these people - the carefully planned attack such as the Manchester suicide bombing, and those using vehicles and knives, which would appear to be more spontaneous."

China strongly condemned the terror attack in London and expressed condolences and sympathy to the victims' families and to the injured, Foreign Ministry spokeswoman Hua Chunying said on Monday.

"China is firmly opposed to any kind of terrorism and is willing to strengthen cooperation with the international community, including the UK, to jointly fight against terrorism and maintain world peace and security," she said.

Hua said there were no reports of Chinese casualties in the London attack and the Chinese embassy in the UK has released alerts to Chinese citizens in the UK to be vigilant.

Police detained 12 people, all from Barking, east of London, in the aftermath of the attack. ISIS claimed responsibility for the attack and said the men were part of a sleeper cell.

On Saturday night, three men, suspected of being Islamist militants, wearing fake suicide vests, drove a white van into pedestrians before running amok with knives in nearby Borough Market, killing seven and injuring 48. They were shot dead by police.

Prime Minister Theresa May said the UK's counterterrorism strategy would be reviewed, as she promised to step up efforts to combat Islamist ideology and work with other countries to prevent the internet being a "safe space" for terrorists.

Writing in the Daily Telegraph, former Counter Terrorism Command chief Richard Walton said: "Islamist extremism won't be solved by intelligence-led investigations. We cannot arrest our way out of the problem. ... We must improve our efforts to tackle this pervasive ideology in our midst."

Media reports recently quoted intelligence officials as saying there are an estimated 23,000 jihadists, or Islamic militants, in the UK. British media quoted UK officials as saying 500 possible plots were being closely monitored. In addition, of the 850 estimated Britons who went overseas to fight for ISIS, some 400 are believed to have returned to Britain, according to UK media reports.

chris@mail.chinadailyuk.com