Police have confirmed that one international student and four British nationals had been detained under the Terrorism Act for allegedly plotting violence as world leaders gather in the UK capital for tomorrow's G20 summit.

Police officers stand next to a sign near the ExCeL centre in London March 31, 2009. [Chinadaily.com.cn via agencies]

A police officer, who confirmed the news with China Daily on Monday night, refused to elaborate on the detentions. But the anonymous officer added there would be a larger security operation surrounding the G20 event and related demonstrations as activists flood London.

The three men, aged 25, 19 and 16, and the 20-year-old woman - all of whom live in the Plymouth area - remain in custody, media reported.

The London-based Guardian newspaper quoted unnamed sources as naming 25-year-old Andrew Sprague as one of the suspects who lived in an apartment where police found homemade firearms, explosives fashioned from fireworks and "political literature".

Police released one of the women, also aged 20 and from Plymouth, late on Monday night after a hearing at Plymouth Magistrates' Court, the Guardian reported.

Police have been hesitant to confirm the detainees were planning an attack during the G20 summit, media have said.

The Guardian quoted Devon and Cornwall assistant chief constable Paul Netherton as saying of the literature: "It's political it relates to political organizations; it's not extreme but it's a different political view."

It also quoted Netherton as saying of the weapons: "These are imitation firearms. The other devices were made of simple fireworks. They're probably not lethal."

A man stands underneath an umbrella while holding onto a banner during a demonstration against the economic crisis and the upcoming G20 meeting of world leaders in Madrid, March 28, 2009. [Chinadaily.com.cn via agencies]

Thousands of activists from across Europe have arrived in London, and French police estimated 1,000 activists will travel to the city from France.

But the London Metropolitan Police Service said it is experienced in policing events and demonstrations, and has dealt with more than 4,500 annually.

"We are monitoring all information relating to planned protests and advertised actions," the unnamed woman officer told China Daily.

"We will have officers deployed at key locations plus a large number in reserve that can respond to deal with whatever takes place."

As police increase security in the run-up to Thursday's summit, people living near the ExCeL center, where leaders will meet, have been directed to carry photo identification.

London-based risk-analysis company Exclusive Analysis told China Daily on Monday that London would be "safe" around the G20 summit despite the possibility more groups will join the protests on Wednesday and Thursday.

"We insist on our analysis unveiled last week, and there should be no large-scale violence in London around the summit," Exclusive Analysis division chief Pepe Egger said.

"We have found that the media have played up the small-scale violence, but the city is basically safe."

(China Daily April 1, 2009)