The UK threat level has been reduced from critical, which means an attack is imminent, to severe, which means an attack is highly likely, Theresa May has said.

The threat level has been lowered in response to fresh arrests and raids linked to Monday's suicide bombing.

Troops will be gradually withdrawn from the streets from Monday onwards, having been drafted in to bolster police numbers, the Prime Minister said.

The Joint Terrorism Analysis Centre (JTAC) increased the threat level to "critical" - its highest level - meaning a further terror attack was considered "imminent".

Speaking after a meeting of the government's Cobra emergency committee, she said the decision had been taken after "a significant amount of police activity" over the last 24 hours.

She said: "The public should be clear about what this means - a threat level of severe means an attack is highly likely. The country should remain vigilant."

The massacre, in which 22 people were killed, was the worst terrorist atrocity to hit Britain since the July 7 attacks in London in 2005.

The de-escalation came as a street in Manchester's Moss Side was evacuated by counter-terror officers.

Boscombe Street was said to have been cleared on Saturday morning, with one witness describing a bomb-disposal van parked at the junction with Yew Tree Road.

An address in the area was being searched by detectives as they sought to close the net on the suspected terror cell behind Manchester Arena suicide bomber Salman Abedi.

Yamma Wu, 29, said she had been ordered not to leave her house by officers. "I can see police cars outside the street and they are not allowing people out or in and there is an evacuation in this area, but because I have got a little baby with me they told me I could say inside, but I could not go out," she said.

The operation in the early hours of Saturday morning brings the number of suspects being held in custody to 11.

The development came as a vast security operation gets under way to protect hundreds of spring bank holiday events across Britain this weekend.

GMP said officers made the latest arrests as they executed a warrant at an address in Cheetham Hill, north of the city centre.

It followed searches at a separate property in Cheetham Hill and an address in the Longsight area in south Manchester.

Residents described hearing a "loud bang" as the latest raid and a controlled explosion took place in Brideoak Street in Cheetham Hill..

Shakufta Bi, who lives across the road, said the noise woke her in the early hours and she saw the street full of police cars and vans along with men in green military uniforms.

Mrs Bi said a Libyan family live at the address - a mother, three sons in their late teens or early 20s and a daughter. The father, an engineer, lives in Dubai and visits occasionally.

All the sons pray at a mosque 200 yards away, known locally as the "Libyan" mosque.

Some neighbours report seeing the sons wearing both "western" dress and traditional Islamic robes.

Another neighbour, Val Jones, said: "I heard a loud bang, it sounded like a door getting kicked in. Then I looked out the window. I seen a lot of police going to the bottom end then I seen a convoy of cars, just normal, private, unmarked cars from the bottom of the street to maybe where that taxi is.

"Car doors flew open, police come out, soldiers in combat gear come out (wearing) green body armour.

"They were just walking up and down the street with guns, focusing (on) ... I don't know which house it was but it was pitch black at that time.

"I knew it was serious but I didn't know it was anything. I thought it was a drugs raid or something like that. You hear a lot of that around here."

Manchester explosion in pictures Show all 37 1 /37 Manchester explosion in pictures Manchester explosion in pictures People running down stairs as they attempt to exit the Manchester Arena after a blast, where U.S. singer Ariana Grande had been performing, in Manchester Twitter/@ZACH_BRUCE/ via REUTERS Manchester explosion in pictures Helpers attend to people inside the Manchester Arena after a suspected suicide bomber detonated an explosive device at the end of an Ariana Grande concert, killing 22 people PA wire Manchester explosion in pictures Armed officers guard outside a hotel near the Manchester Arena following reports of an explosion, in Manchester, Britain EPA Manchester explosion in pictures Police officers are seen outside the Manchester Arena, where U.S. singer Ariana Grande had been performing, in Manchester, northern England Reuters Manchester explosion in pictures MANCHESTER, ENGLAND - MAY 23: Police escort members of the public from the Manchester Arena on May 23, 2017 in Manchester, England. An explosion occurred at Manchester Arena as concert goers were leaving the venue after Ariana Grande had performed. Greater Manchester Police have confirmed 19 fatalities and at least 50 injured. (Photo by Christopher Furlong/Getty Images) Getty Images Manchester explosion in pictures MANCHESTER, ENGLAND - Emergency services arrive close to the Manchester Arena on May 23, 2017 in Manchester, England. There have been reports of explosions at Manchester Arena where Ariana Grande had performed this evening. Greater Manchester Police have have confirmed there are fatalities and warned people to stay away from the area. (Photo by Dave Thompson/Getty Images) Getty Images Manchester explosion in pictures MANCHESTER, ENGLAND - MAY 23: Police escort members of the public from the Manchester Arena on May 23, 2017 in Manchester, England. An explosion occurred at Manchester Arena as concert goers were leaving the venue after Ariana Grande had performed. Greater Manchester Police have confirmed 19 fatalities and at least 50 injured. (Photo by Christopher Furlong/Getty Images) Getty Images Manchester explosion in pictures Medics deploy at the scene of a reported explosion during a concert in Manchester, England on May 23, 2017. British police said early May 23 there were "a number of confirmed fatalities" after reports of at least one explosion during a pop concert by US singer Ariana Grande. Ambulances were seen rushing to the Manchester Arena venue and police added in a statement that people should avoid the area / AFP PHOTO / Paul ELLIS (Photo credit should read PAUL ELLIS/AFP/Getty Images) AFP/Getty Images Manchester explosion in pictures Police and fans close to the Manchester Arena, after reports of an explosion Getty Images Manchester explosion in pictures There have been reports of explosions at Manchester Arena where Ariana Grande had performed Getty Images Manchester explosion in pictures Police deploy at scene of explosion in Manchester, England, at a concert in Manchester Arena AFP/Getty Images Manchester explosion in pictures Police stand by a cordoned off street close to the Manchester Arena Getty Images Manchester explosion in pictures Police deploy at scene of explosion in Manchester, England AFP/Getty Images Manchester explosion in pictures Police deploy at scene of a reported explosion during a concert in Manchester, England, on May 23, 2017. British police said early May 23 there were "a number of confirmed fatalities" after reports of at least one explosion during a pop concert by US singer Ariana Grande. Ambulances were seen rushing to the Manchester Arena venue and police added in a statement that people should avoid the area AFP/Getty Images Manchester explosion in pictures An ambulance drives away from the Manchester Arena, where U.S. singer Ariana Grande had been performing, in Manchester Reuters Manchester explosion in pictures Police escort members of the public from the Manchester Arena in Manchester, England. Getty Images Manchester explosion in pictures A woman sits in the street in a blanket near the Manchester Arena as police guard the area following reports of an explosion, in Manchester, Britain EPA Manchester explosion in pictures Two women wrapped in thermal blankets stand near the Manchester Arena, where US singer Ariana Grande had been performing, in Manchester Reuters Manchester explosion in pictures A Royal Logistic Corps (RLC) bomb disposal robot is unloaded outside the Manchester Arena following reports of an explosion, in Manchester. At least 19 people have been confirmed dead and others 50 were injured, authorities said. It is being treated as a terrorist incident until police know otherwise EPA Manchester explosion in pictures A Royal Logistic Corps (RLC) bomb disposal robot is unloaded outside the Manchester Arena following reports of an explosion, in Manchester EPA Manchester explosion in pictures Members of the public receive treatment from emergency service staff at Victoria Railway Station close to the Manchester Arena on May 23, 2017 in Manchester, England. There have been reports of explosions at Manchester Arena where Ariana Grande had performed this evening. Greater Manchester Police have have confirmed there are fatalities and warned people to stay away from the area Getty Images Manchester explosion in pictures Armed police after a suspected terrorist attack at the Manchester Arena at the end of a concert by US star Ariana Grande left 19 dead PA wire Manchester explosion in pictures Emergency services arrive close to the Manchester Arena in Manchester Getty Images Manchester explosion in pictures An amoured police vehicle patrols near Manchester Arena in Manchester Getty Images Manchester explosion in pictures A man carries a young girl on his shoulders near Victoria station in Manchester AFP/Getty Images Manchester explosion in pictures Police officers stand at the Miller Street and Corporation Street Crossroads, in front of the Manchester Arena in Manchester, England Getty Images Manchester explosion in pictures Police block a road near to the Manchester Arena in central Manchester, England AP Manchester explosion in pictures Armed police patrol near Victoria station in Manchester, northwest England. Twenty two people have been killed and dozens injured after a suspected suicide bomber targeted fans leaving a concert of US singer Ariana Grande in Manchester Getty Images Manchester explosion in pictures Police forensic officers leave the Manchester Arena as they investigate the scene of an explosion in Manchester Getty Images Manchester explosion in pictures A forensic officer collects evidence on a walkway between Victoria station and Manchester Arena following a deadly terror attack in Manchester, Getty Images Manchester explosion in pictures A woman and a young girl wearing a t-shirt of US singer Ariana Grande talks to police near Manchester Arena following a deadly terror attack in Manchester, Getty Manchester explosion in pictures epaselect epa05982725 Susan Walton and daughter Katie, 10 (pictured), who attended the concert of Ariana Grande at the Manchester Arena, are seen in Manchester, Britain, 23 May 2017. According to a statement released by the Greater Manchester Police on 23 May 2017, police responded to reports of an explosion at Manchester Arena on 22 May 2017 evening. At least 19 people have been confirmed dead and around 50 others were injured, authorities said. The happening is currently treated as a terrorist incident until police know otherwise. According to reports quoting witnesses, a mass evacuation was prompted after explosions were heard at the end of US singer Ariana Grande's concert in the arena. EPA/NIGEL RODDIS EPA Manchester explosion in pictures Mayor of Greater Manchester Andy Burnham and Manchester City Council Leader Sir Richard Leese speak to the media outside Manchester Town Hall after a suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester Dave Higgens/PA Wire Manchester explosion in pictures The media gather behind a police cordon in Manchester Getty Images Manchester explosion in pictures Flowers left close to the Manchester Arena, the morning after a suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester Danny Lawson/PA Wire Manchester explosion in pictures Ariana Grande concert attendees Karen Moore and her daughter Molly Steed, aged 14, from Derby, leave the Park Inn where they were given refuge after last night's explosion at Manchester Arena Getty Manchester explosion in pictures Signs saying 'We love Manchester' are displayed in a window in Manchester, England Getty Images

On Friday evening, a 44-year-old man was arrested in the Rusholme area of the city. Witnesses reported seeing armed police storm a bus on its way to the town centre at around 7pm.

Twenty-two people, including seven children, were killed and scores injured when Abedi launched a suicide attack at Manchester Arena on Monday night.

The Duke of Cambridge and Labour leader Jeremy Corbyn are expected to head to Wembley on Saturday.

In Hull, US star Katy Perry will perform amid tight security at Radio 1's Big Weekend, while in Manchester, armed officers will patrol The Courteeners' concert at Old Trafford cricket ground, where fans were warned extra security checks will be in place.

On Sunday, the Great Manchester Run will go ahead as planned, with defiant runners and spectators due to turn out in large numbers as the city recovers from the atrocity.

Activity in the Manchester probe is expected to continue throughout the weekend and investigators remained at at least 12 locations across the North West on Friday evening.