Hundreds of troops flooded the streets yesterday amid fears a terrorist bomb cell remains on the loose.

With the threat level raised to 'critical', up to 1,000 heavily-armed military personnel were guarding national landmarks, the Royal Family and nuclear sites.

Soldiers carrying SA-80 rifles walked alongside uniformed officers outside the Houses of Parliament, Buckingham Palace, Downing Street and other important sites.

Show of strength: Armed forces patrol on the streets outside Parliament which was closed to the public yesterday

The operation freed hundreds of police marksmen to join the manhunt for suspected accomplices of Salman Abedi.

Police commanders fear he was at the centre of an Islamic State-inspired network, including a master bombmaker.

The deployment came amid a huge lockdown, with tours and other events called off at the Palace of Westminster, which closed to the public.

The changing of the guard in the Mall was cancelled, along with a victory parade by Premier League champions Chelsea.

A man with a knife was arrested by eight officers outside Buckingham Palace. In the City, a square adjacent to St Paul's Cathedral was briefly shut down.

Protection: A team of soldiers carrying rifles march along Downing Street- just one of the national landmarks where military personnel have been stationed

Manchester remained on edge, with shoppers screaming in panic as they were evacuated from the city's Arndale centre. Police arrested a man there after reports a suspicious package had been found. He is not linked to the terrorist inquiry.

The operation to put armed forces on the streets was authorised by Defence Secretary Sir Michael Fallon at the request of police. It followed a emergency Cobra briefing in which Prime Minister Theresa May was warned another attack remains 'imminent'.

Sir Michael said 2,800 troops were on standby for patrols with police on the streets, at rail stations and at sports events if needed. He did not rule out soldiers being deployed to Wembley for this weekend's FA Cup final or to Twickenham for the Premiership rugby final if police felt 'stretched', but stressed the military had not yet been asked to do this.

He said the 980 troops already deployed would 'free up more armed police from static guarding duties' such as at foreign embassies.

Joint operation: A police officer directs troops at Buckingham Palace. The operation frees officers to continue pursuing accomplices of the bomber

Soldiers entering the Palce of Westminster. Soldiers will take on patrol duties under police command

'Our troops have been on standby ready to do this since the attacks in other European cities,' he added.

He said there were additional troops in reserve in case police wanted more.

Experts at the Joint Terrorism Analysis Centre raised the threat level to 'critical' – the highest on the scale – as police worked to identify further conspirators.

The decision to put troops on the streets is part of Operation Temperer, drawn up more than two years ago by top civil servants, police chiefs and military brass.

It is the first time such a step has been taken since Tony Blair sent tanks to Heathrow Airport in 2003.

The sight will stir memories of the Troubles in Northern Ireland, where soldiers were deployed from August 1969 until July 2007.

The threat level has been raised to 'critical' only twice since the system was introduced in 2006 – when a plot to smuggle liquid explosives on transatlantic planes was discovered, and when police hunted two men who attacked Glasgow airport.

Standing guard: A soldier outside the door of Number Ten yesterday, as hundreds were deployed across the city

Metropolitan Police commissioner Cressida Dick declined to say how long the military will remain

Extra police were seen across the rail network yesterday and teams patrolled the Thames. Armed officers were stationed around St Paul's Cathedral, which hosted the Queen and Duke of Edinburgh at a service marking the centenary of the Order of the British Empire.

Metropolitan Police commissioner Cressida Dick declined to say how long the military will remain.

She said there were a 'number of unknowns' and security services needed 'a better understanding' of the situation before the threat level could be reduced.

Police Federation chairman Steve White said the Government must ensure there is a 'resilient, fully-resourced police service', adding: 'We the police simply do not have the resources to manage an event like this on our own.'