London (CNN) -- In the wake of an alleged terrorist attack on one of its soldiers, Britain is forming a task force that will examine the dynamics behind extremist groups in the country, Prime Minister David Cameron's office announced Sunday.
The group, led by Cameron, will "have a general focus on extremist groups, but accept that in practice the greatest threat is from Islamist extremists," a statement from Downing Street said.
The Muslim Council of Britain said the task force needs to look at "extremism from all quarters" while forming an effective strategy.
"In doing so, we hope wisdom prevails as we reflect on the response of these past few days and the missed opportunities of previous years," said a statement from the council's secretary-general, Farooq Murad. "We must be vigilant and ensure we do not inadvertently give into the demands of all extremists: making our society less free, divided and suspicious of each other. Lessons from the past indicate that policies and measures taken in haste can exacerbate extremism."
The brutal slaying Wednesday of British soldier Lee Rigby near the Royal Artillery Barracks in a working-class neighborhood in southeast London shocked people across the United Kingdom.
It also triggered a far right-wing street demonstration Saturday in northern England, in which irate protesters called for Muslims to leave Britain.
Late Sunday, a mosque caught fire in the city of Grimsby on the country's east coast, and police arrested two men on suspicion of arson. The blaze coincided with inciting messages on social media "by a small minority of individuals," according to Humberside County police.
"Those people should be aware that we are monitoring these sites in Humberside and we will take action against those intent on attempting to incite violence or post messages of a racial nature," police warned.
Nine people have been arrested in connection with the Woolwich attack, including three men who were taken into custody Saturday and another on Sunday. The three arrested Saturday were released early Monday on bail, police said.
The men were being held on suspicion of conspiracy to commit murder, the Metropolitan Police said in a statement.
Police did not detail how they were allegedly tied to the killing nor did they release their identities, saying only that the men -- ages 21, 22, 24 and 28 -- were arrested by detectives from the Counter Terrorism Command and taken to a south London police station.
Two men arrested at the scene of the killing are now under guard in South London hospitals after being shot in a confrontation with police.
Britain's MI5 intelligence agency will report the results of a preliminary investigation into the suspects this week to the Intelligence and Security Committee, government sources said. The report is expected to be confidential.
Two women who were arrested Thursday were released shortly thereafter without charge.
Addressing the crowd Saturday in Newcastle, far-right English Defence League leader Tommy Robinson said that Rigby's killing should be a wake-up call to British citizens.
"We cannot allow this soldier's death to be in vain," Robinson said. "We are the only ones who dare say it. When did the truth become hate speech?"
Police estimated that between 1,500 and 2,000 people attended the rally. A counter demonstration drew a few hundred people, police said.
Rigby's family visited the scene of his death on Sunday.
CNN's Erin McLaughlin contributed to this report.