Soldiers were deployed on the streets of London on Wednesday, as police hunted down the network behind the Manchester suicide bomber and authorities warned another attack was “imminent.”

Manchester’s police chief, Ian Hopkins, said it’s “very clear” that police are investigating a “network” responsible for the attack that killed 22 people including children at a pop concert in the northern city on Monday night. While police search for accomplices, soldiers were deployed to patrol key potential targets in London such as the Houses of Parliament and Prime Minister Theresa May’s office in Downing Street.

May, who has suspended campaigning for the general election next month, announced late on Tuesday that intelligence analysts had raised the U.K. terrorism threat to “critical” from “severe” for the first time since 2007. It’s the highest threshold and means an attack is imminent.

At the black iron gates of Downing Street, a young soldier in camouflage fatigues stood guard, an assault rifle in his hands and his combat helmet hanging at his waist. In Westminster’s New Palace Yard — where a lone-wolf assailant killed a policeman in March after mowing down pedestrians — a Metropolitan Police officer armed with a submachine gun chatted to his new colleague from the Parachute Regiment.

The worst terrorist incident on U.K. soil since 2005 came just over two weeks before a general election and ahead of a weekend that has major sporting events scheduled including soccer’s FA Cup final at the national stadium in Wembley, northwest London. Home Secretary Amber Rudd said a total of 984 soldiers had been drafted in to help police in London and elsewhere.

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The BBC reported that the bomber is thought to have been using a device built by someone else. Police arrested three men in south Manchester on Wednesday in connection with the attack. They had arrested a 23-year-old man, reported to be the bomber’s brother, on Tuesday.

Ramadan starts this weekend, and Islamic State, which claimed the Manchester attack, has traditionally encouraged followers to carry out attacks during the holy month.

As soldiers patrolled at the Houses of Parliament, all tours of the complex were canceled, while the Bank of England’s museum was closed. The changing of the guard at Buckingham Palace, a popular tourist attraction, was also canceled. Chelsea Football Club ditched a victory parade planned for the weekend.

Campaigning for the June 8 general election remained on hold, though the U.K. Independence Party said it plans to end the truce and publish its manifesto Thursday, while the main opposition Labour Party said it will resume its campaign on Friday.

Submachine Guns

The biggest impact for Britons going about their lives is the military deployment. Rudd said as many as 3,800 troops would be available to meet requests from the police. They will be stationed in places where Britain usually has armed police, freeing those officers for other front-line patrol duties. Most U.K. police are unarmed, so the most jarring sight for many British people may be that of officers with submachine guns on the streets.

During the 2012 Olympics, rocket batteries were set up around London as a precautionary measure against an airborne attack. In 2003, armored vehicles were briefly sent to Heathrow Airport because of a specific terrorist threat. But historical precedents for large-scale deployment on the British mainland are few. Apart from World War II, when the country braced for an invasion, there is the General Strike of 1926, when soldiers protected food supplies from protesters.

France has had a state of emergency in place since the Charlie Hebdo terrorist attacks in January 2015, giving the state powers to ban public gatherings and heighten surveillance. President Emmanuel Macron’s government will have to decide whether to extend the state when current powers run out in mid-July. About 7,000 soldiers are currently deployed on patrols in mainland France as part of the counter-terrorist effort. Belgium has also deployed troops on the streets since last year’s attacks on Brussels airport and the city’s Metro.