MANCHESTER, England >> Britain reduced its terrorism threat level a notch, from “critical” to “severe” on Saturday, as authorities said they had made major progress in unravelling the plot behind the Manchester concert bombing.

But police said more arrests were expected, and security was tight across Britain as hundreds of thousands of people attended major soccer matches, concerts and other big holiday-weekend events.

Police made two more arrests in Manchester on Saturday on suspicion of terrorism offenses, bringing the number of suspects in custody to 11. All are men, aged between 18 and 44. In addition, Manchester bomber Salman Abedi’s father and younger brother are in detention in Libya.

British Prime Minister Theresa May said “a significant amount of police activity” and several arrests had led to the level being lowered. But she urged Britons to remain vigilant and said soldiers would remain at high-profile sites throughout the holiday weekend. The troops will gradually be withdrawn beginning on Monday, she said.

A severe threat means an attack is “highly likely,” according to the scale set by Britain’s Joint Terrorism Analysis Centre. Until it was raised Tuesday, it had stayed at severe since mid-2014.

Assistant Commissioner Mark Rowley, Britain’s top counterterrorism police officer, said authorities have dismantled a “large part” of the network around Abedi, who killed 22 people and wounded dozens by bombing an Ariana Grande concert Monday in Manchester.

But he said there were still “gaps in our understanding” of the plot, as investigators probed Abedi’s potential links to jihadis in Britain, Europe, Libya and the Middle East.

Rowley said the investigation had made “rapid progress,” and police “are getting a greater understanding of the preparation of the bomb.”

“There is still much more to do. There will be more arrests and there will be more searches,” he said.

Abedi, a 22-year-old Briton of Libyan descent who grew up in Manchester, died in Monday’s explosion.

Police used an explosive device Saturday to get into a property in Manchester to arrest two men, aged 20 and 22. Investigators have searched 17 properties, including Abedi’s home in south Manchester and other houses in nearby districts.

Residents were evacuated from streets in the south Manchester neighborhood of Moss Side in what police called a precaution as one search was carried out Saturday. Photos showed an army bomb-disposal unit at the property.

One search was at an apartment in a Manchester high-rise that British media say was rented by Abedi in the months before the attack. Mohammed El-Hudarey, a friend of the landlord, said after Abedi moved out about six weeks ago there was a strong smell of chemicals and debris, including metal rods and cut-up fabric.

“We thought he must have been a drug dealer or doing witchcraft,” El-Hudarey told the BBC.

Britain’s health service said Saturday that 63 people injured in the bombing remained hospitalized, 20 of them in critical condition. A total of 116 people were treated in hospitals after the bombing.

Despite the alert, police have urged people to go out and enjoy themselves over the three-day holiday weekend.

Hundreds of soldiers replaced police at high-profile sites including Buckingham Palace and Parliament, and police armed with submachine guns have been deployed in city centers, transit hubs, tourist areas and major events.

Armed police were on the streets Saturday outside London’s Wembley Stadium, and security guards conducted extra bag checks, as 90,000 fans arrived for the FA Cup soccer final between Chelsea and Arsenal, one of the biggest sporting events of the year.

Before kickoff, Prince William laid a wreath in memory of the victims alongside Manchester mayor Andy Burnham.

Arsenal fan Liz Johnson says she was a “sad after what happened in Manchester. But I grew up in Ireland, so bombs did go off there and life does go on.”

“We will be thinking about all the people who died and were injured,” she said.

“Everything should go on as normal,” said Chelsea fan Steve Barrass, who came from Manchester to watch the FA Cup in London with his son.

A security alert at London’s Old Vic Theatre prompted the evacuation of cast and audience in the middle of a performance of “Woyzeck” — starring “Star Wars” actor John Boyega. Police searched the building and found nothing suspicious.

Manchester slowly returned to normal, though the damaged arena and adjacent Victoria train station remained closed.

Grande promised to return to “the incredibly brave city of Manchester” for a benefit concert to raise money for the bombing victims and their families.

“Our response to this violence must be to come closer together, to help each other, to love more, to sing louder and to live more kindly and generously than we did before,” the American singer said. “We will not quit or operate in fear. We won’t let this divide us. We won’t let hate win.”