Britain reduced its terror threat level from "critical" to "severe" Saturday after fresh raids and arrests by police investigating the Manchester Arena bombing after an Ariana Grande concert.

The change indicates another terrorist attack is highly likely, rather than imminently expected.

Prime Minister Theresa May said soldiers who have been patrolling streets would be withdrawn gradually from midnight Monday. She said "significant" activity by police in recent hours had prompted the decision.

More from NBC News:

Kushner met with Russian banker who is Putin crony, spy school grad

Why Trump's silence on NATO is a big deal

Hillary Clinton attacks Donald Trump at Wellesley graduation

Britain's most senior counterterrorism officer, Mark Rowley, said 11 men are now in custody in connection with the suicide bombing, which authorities say was carried out by Manchester-born Salman Abedi. The suspected bomber's motives remain unclear.

"We are getting a greater understanding of the preparation of the bomb," Rowley said. "This greater clarity and this progress has led (the Joint Terrorism Analysis Centre) — the independent body that assess threat — to come to the judgment that an attack is no longer imminent."

Security fears were heightened as Britain enjoys a holiday weekend with several high-profile music festivals and sporting events that draw large crowds. Rowley said high level of policing will remain in place for "practical and precautionary reasons," and advised people to remain vigilant.

"Once we get past the weekend, we will be looking to step down the extra resources we have had in place over the last week," he said. "And the military support we have had in place over the past few days, under Operation Temperer, we will start to phase out as well."

Two men were arrested early Saturday in a raid on a home in the Cheetham Hill area of Manchester after officers used a controlled explosion to gain entry.

Some hours later, police cordoned off a large area in the Moss Side area and houses were evacuated with a bomb disposal unit sent to the scene.

Political campaigning for Britain's June 8 national election, which was suspended after the Manchester attack, resumed Friday with the bombing becoming a central issue.