Friends of British hostage Norman Kember have held an hour-long silent vigil for him, as diplomats in Iraq hunt for clues of his whereabouts.
Norman Kember has been blindfolded and shackled
A group called the Swords of Truth say they abducted Mr Kember, 74, and three co-workers in Baghdad last month.
They said they would kill the captives last Saturday if their demands were not met - but nothing further has emerged.
Those at the vigil in Trafalgar Square carried candles and rainbow flags bearing the message "peace".
"This vigil is really to keep the four men who have been taken hostage in the public consciousness," said one of the organisers, Pat Gaffney.
"And to remind those who are holding them that they are not forgotten."
The government said it was doing all it could to ensure the captives were released unharmed, but the Foreign Office said on Monday there had been no further developments.
Mr Kember, from Pinner, north London, American Tom Fox, 54, and Canadians James Loney, 41, and Harmeet Singh Sooden, 32, had travelled to Iraq as a "gesture of solidarity" with Canada-based international peace group Christian Peacemaker Teams.
The hostage-takers, who want all prisoners in Iraq to be released, have accused the men of spying, although CPT denied this.
Friends and family of Mr Kember have endured an agonising wait for news.
Chris Cole, director of the Fellowship of Reconciliation, of which the pensioner is a trustee, said: "We are looking for a miracle at this stage."
Bruce Kent, a friend of Mr Kember for 15 years, said: "I haven't given up hope at all.
"It's quite possible they may be preparing a video to say why they are releasing them."
Muslim leaders in Britain and abroad - as well as terror suspects held in the UK - have repeatedly called for the release of Mr Kember, who was seen on video shackled and blindfolded.
They also pleaded for the other three hostages to be freed.