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The 22-year-old, thought to be Michael Adebowale, from Greenwich, was taken into custody at a south London police station this afternoon, Scotland Yard confirmed.
He was arrested with 28-year-old Michael Adebolajo after Drummer Rigby was murdered near Woolwich barracks on May 22.
The two suspected Islamic terrorists were shot by armed police after they were called to the scene in the immediate aftermath of the murder.
Adebowale was also arrested this afternoon on suspicion of the attempted murder of a police officer.
It has since emerged that both men were known to MI5 before the attack.
Since the death of father of one Lee Rigby, 25, ten people have been arrested. As well as the two men, a 50-year-old man was held in Welling, south-east London on Monday and a 22-year-old man was arrested in Highbury, north London, on Sunday. Three men held on Saturday have been released on bail, as has a fifth man, aged 29.
Two women, aged 29 and 31, were arrested on suspicion of conspiracy to murder but were later released without charge.
The murder has sparked activity from far right group the English Defence League, which saw more than 1,000 supporters march to Downing Street chanting "Muslim killers off our streets" and "There's only one Lee Rigby" on Bank Holiday Monday. A massive police presence kept them separate from a smaller group of anti-fascist activists, with officers making 13 arrests in total for a range of public order offences.
Counter terrorism police are also investigating an attack on a prison warden by alleged fanatical Islamic inmates, believed to have been inspired by the murder.
The family of Adebolajo have expressed their "profound shame and distress" over the killing.
A statement released by the family said: “Nothing we can say can undo the events of last week. However, as a family, we wish to share with others our horror at the senseless killing of Lee Rigby, and express our profound shame and distress that this has brought to our family.
"We send our heartfelt condolence to Lee Rigby's family and loved ones."
"We wish to state openly that we believe that there is no place for violence in the name of religion or politics. We believe that all right thinking members of society share this view wherever they were born and whatever their religion and political beliefs.
"We wholeheartedly condemn all those who engage in acts of terror and fully reject any suggestion by them that religion or politics can justify this kind of violence.”