The brother of hostage Ken Bigley cancelled a speech to a Stop The War rally in Liverpool but sent a message to the gathering of more than 200.
Rose Gentle's son, Gordon, was killed in Iraq in June
Paul Bigley was due to speak by phone link from Amsterdam on Friday.
Instead he sent a message read by Carmel Brown of Stop The War, which said Prime Minister Tony Blair had "blood on his hands."
Mark Holt, Chair of Merseyside Stop The War said: "Our thoughts and our prayers are with the family of Ken Bigley."
Mr Bigley withdrew in the light of the confirmation his brother had been killed by his kidnappers.
Paul Bigley's statement was read out at the meeting
A minute's silence was held for Ken Bigley and all who have perished in Iraq.
Speakers included Azmat Begg, whose son Moazzam, 36, is a detainee at the US base in Guantanamo Bay.
Also speaking was Rose Gentle whose son Gordon, 19, from Glasgow, was killed while serving as a soldier in Iraq. She has urged Tony Blair to meet dead soldiers' families.
Mrs Gentle told BBC Online: "He (Ken Bigley) should have been free ages ago.
"I think the best thing to do is for Tony Blair to resign."
Paul Bigley has already called for people to back the national Stop The War demonstration due to take place on Sunday, 17 October.
He said: "The more people raise their voices, the safer we will all be."