Pro-Palestinian campaigners wept as they spoke of their fears about the fate of British citizens and others aboard the flotilla of ships carrying aid to Gaza.
Hundreds of activists blocked Whitehall shouting "Free Palestine" and carrying flags and banners with slogans such as "Stop Israel's War Crimes in Gaza" and "End the Criminal Siege of Gaza".
Kate Hudson, chairwoman of the Campaign for Nuclear Disarmament (CND), who joined the demonstration, said she knows Sarah Colborne of the Palestine Solidarity Campaign, among others on board the flotilla.
Brushing aside tears, she said they had not heard from her, or Kevin Ovenden, another pro-Palestine activist, or Ismail Patel, another member of the flotilla and chairman of the Palestinian rights group Friends of Al-Aqsa.
She said: "Obviously we have great support for the humanitarian convoy which has gone there to try and bring relief to the people in Gaza. It is devastating and deplorable that the Israeli forces have attacked civilians on the flotilla.
"We have close friends on the boat on which people were killed and we are here waiting for news. We are trying to get through to them but we are not getting any answers."
Maha Rahwanji, 35, the daughter of Palestinian exiles, who lives in London and works in adult education, also shed tears as she attended the demonstration.
"Today marks the anniversary of my father's death. Instead of being at a memorial service, after he died 20 years ago for the cause, instead of being at his graveside to remember him, I am mourning the loss of another 20 people," she said. "What has happened to the flotilla is just heartbreaking."
Also present on the demonstration was journalist and author Victoria Brittain, a patron of the Palestine Solidarity Campaign (PSC).
She said: "The Israelis have killed 20 peaceful people trying to deliver food, medicine and building materials, wheelchairs and dental equipment to people who have been under siege in Gaza for far, far too long."