Police have described the murder of a charity worker in south London as "a brutal attack on a defenceless man".
Abdel Salam Hussain Abdel Salam, 56, was found dead at his home in Boone Street in Lewisham on Saturday morning.
A port-mortem examination showed the Sudanese man died from blood loss after being stabbed in the leg.
"Abdel Salam had devoted much of his life combating abuse of human rights upon others," said Det Ch Insp Damian Allain, who is leading the inquiry.
The victim volunteered at the Redress charity, a human-rights organisation which supports survivors of torture, Mr Allain added.
"Abdel Salam was a large man, who used two sticks to walk, as he had some difficulty," he said.
"He would often travel by minicab or bus to take him to the charity where he volunteered in central London, and many people would have seen him on his route."
He was last seen alive at home on Friday afternoon and police are trying to establish what he did between then and Saturday morning.
No arrests have been made.