London, May 24: Prime Minister Theresa May has placed Britain on virtually a war footing following the Manchester bombing, warning that another terror attack was "imminent", raising the security to its highest level and calling out the army.

An international dragnet has so far found six suspects, including a woman who was picked up in Manchester. "I think it's very clear that this is a network that we are investigating," police chief Ian Hopkins said outside Manchester police headquarters.

Among the 22 killed and 64 injured were several children, who had gone to see their favourite American pop singer, Ariana Grande.

Another terror attack is "imminent", the Prime Minister warned, raising the threat level from "severe" to "critical" - which is the highest. She has also called out the army which has started taking over from police in guarding key installations and crowded events. That includes this summer's cricket. The Champions Trophy begins on June 2.

Armed police at railway stations in London are now a familiar sight but they are being joined today by 1,000 troops. Another 2,800 troops are being deployed elsewhere in Britain.

This morning's Changing the Guard ceremony at Buckingham Palace was cancelled to free up police officers for more urgent duties. At Windsor Castle, too, Changing the Guard was cancelled. The public are also now barred from the Palace of Westminster.

A statement on the Parliament website said all tours and events were being cancelled with immediate effect.

May said last night that it had been concluded "that the threat level should be increased for the time being from severe to critical". "This means that their assessment is not only that an attack remains highly likely but that a further attack may be imminent."

There is a strong suspicion that Salman Abedi, the 22-year-old suicide bomber of Libyan origin, did not act alone but was part of a cell. Abedi had returned very recently from a trip to Libya.

Manchester police made five new arrests and searched an address in the city centre. A source said British investigators were hunting for anyone who may have helped build the suicide bomb and who could be ready to kill again.

Around 9pm local time, the police announced the arrest of a sixth person. The arrest of the woman followed an armed raid on a block of flats in the north of the city, the BBC reported.

Police in Tripoli, the Libyan capital, arrested a brother of Abedi and his father. A spokesman for the local counter-terrorism force said younger brother Hashem Abedi was arrested on suspicion of links with the Islamic State and was suspected of planning to carry out an attack in Tripoli.

The bomb used in the attack appeared to contain carefully packed shrapnel and have a powerful, high-velocity charge, according to leaked photographs from the investigation.

Grande was scheduled to perform two shows at London's O2 arena this week, but the singer's representative said she was suspending her tour to assess the situation and to "pay our proper respects to those lost".

Chelsea soccer club said it had cancelled a victory parade that had been scheduled to take place on Sunday to celebrate its Premier League title.