More than 30 people gathered in Trafalgar Square on Thursday, at a vigil for British hostage Norman Kember, kidnapped in Iraq.
Mr Kember was last seen in a video blindfolded with his hands tied
A group called the Swords of Truth say they abducted the 74-year-old and three co-workers more than a month ago.
Peace activist Mr Kember was last seen in a video released three weeks ago - but nothing further has emerged.
Well-wishers and tourists looked on as the small crowd waved flags bearing the word peace and read prayers aloud.
Both Muslim and Christian organisations joined the one-hour vigil in central London on Thursday afternoon.
Sue Claydon, a friend of Mr Kember, told BBC News she has not given up hope for his safe return.
"We are assuming he is still alive - we are working on the basis of hope and faith.
"Norman, I am sure, is doing exactly the same thing and I know both he and his family and the other hostages and their families will be feeling very supported."
Anas Al-Tikriti, of the Muslim Association of Britain, travelled to Baghdad where he placed adverts in newspapers and appealed on television for the release of Mr Kember.
He believes his appeal was heard by Mr Kember's captors.
"They would have heard who they are holding... a group of friends of Iraqis and the entire world from all spectrums... all want them to be released simply because they are friends of Iraq and its plight."
Mr Kember, from Pinner, north London, was seized in Baghdad on 26 November while travelling in Iraq as a "gesture of solidarity" with Canada-based international peace group Christian Peacemaker Teams.
Demands not met
He was taken captive along with Canadians James Loney, 41, and Harmeet Singh Sooden, 32, and US citizen Tom Fox, 54.
The Swords of Truth group demanded all Iraqis detained by coalition forces be freed by 10 December - a call which has not been met.
Mr Kember was last seen in a video aired on the al-Jazeera television network on 8 December, blindfolded and with his hands tied.
A Foreign Office spokeswoman said there was no new information about Mr Kember but said his family had made appeals on Iraqi radio and in newspapers over the Christmas period.