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British Muslims who speak out against extremism are being warned that they could be targeted by members of the Somali militant group al-Shabab.
British Muslim commentator Mohammed Ansar said he was currently under police protection because of what he was told was a "credible threat".
It comes after al-Shabab militants posted a video online which named Muslims who have criticised jihadists.
Scotland Yard said it was investigating the video.
The hour-long film, narrated by an unidentified man with a British accent, praises acts of Islamist terrorism.
Mr Ansar said he was visited by police and given protection earlier this week.
The imam and broadcaster Ajmal Masroor said he had also had a late night visit from two detectives after he was named in the film.
He said police warned him they had credible evidence of a potential threat against him.
But Mr Masroor told the BBC he would not be silenced and would continue to speak out against terrorism and extremism.
Muslim commentator Mohammed Shafiq tweeted: "I was also visited by the police and told of threats made against me."
Mr Shafiq, from Rochdale, added that the threats had been confirmed by counter-terrorism police and he had been given extra protection.
The executive of the Ramadhan Foundation thanked his followers on Twitter for their support, saying: "Thank you for all the concern, love and support I've received after al-Shabab threats against me. I will never give in to terrorism."
Scotland Yard confirmed it had spoken to a number of individuals and said it was currently assessing the contents of the video.
At least 67 people died when suspected al-Shabab militants stormed Kenya's Westgate shopping centre on 21 September.
Al-Shabab is fighting to create an Islamic state in Somalia and is banned as a terrorist group by both the US and the UK.
The Islamist group, which is believed to have between 7,000 and 9,000 fighters, controls large parts of Somalia.