Police on Monday arrested a 10th suspect in connection with the vicious killing of an off-duty British soldier on a London street in an apparent Islamic extremist attack.

The 50-year-old man was detained in Welling, east of London, on suspicion of conspiring to murder 25-year-old soldier Lee Rigby, Scotland Yard said. Police didn't provide further details about the suspect's identity.

The latest arrest came as around 1,000 supporters of a far-right group marched through central London protesting the killing, clashing with a smaller group of anti-fascist demonstrators.

Police have already taken nine other suspects into custody, including the two main men who were shot and wounded by officers at the scene of Wednesday's killing. Michael Adebolajo, 28, and Michael Adebowale, 22, remain under armed guard in separate London hospitals. Four other men remain in custody at a London police station.

Witnesses say Rigby, who had served in Afghanistan, was walking near his barracks in southeast London's Woolwich area when he was run over by a vehicle and repeatedly stabbed with knives and meat cleavers.

The gruesome scene was recorded on witnesses' cellphones, and a video picked up by British media showed one of the two main suspects, holding a bloody knife and cleaver, making political statements and warning of more violence. Hard-line Muslim leaders have identified the man in the video as Adebolajo.

Kenyan officials have confirmed that Adebolajo was arrested in Kenya in 2010 with five others near the country's border with Somalia. Police believed Adebolajo was going to work with Somali militant group al-Shabab.

The killing in London has fed a spike in anti-Muslim sentiment in Britain, with police and activists reporting a surge in hate crimes, including violence and vandalism.

A mosque in the northern England town of Grimsby was firebombed Sunday night, according to the mosque's chairman, Diler Gharib, who told a local newspaper he was discussing how to thank his neighbors for the support they had shown his community in the past days when the building was hit with gasoline bombs.

Police said they arrested two people in connection with the incident. The fire was extinguished, and no one was injured, they said.