Relatives of Britons caught up in the deadly storming of an aid flotilla by Israel were desperately seeking news amid worldwide diplomatic condemnation and public protests.
Foreign Secretary William Hague said he "deplored" the loss of life in the operation and was seeking more information and "urgent access" to any UK nationals involved.
As many as 19 people on boats carrying 10,000 tonnes of supplies to Gaza in defiance of a three-year blockade were reportedly killed when Israeli commandoes boarded in international waters.
The Foreign Office said it had no information to suggest any Britons are among the dead or injured but was not able to give any more information about how many were on the aid mission.
Israel insisted its forces had fought back against a pre-planned attack on them using weapons including knives and guns but activists said the commandoes were shooting as they boarded.
Mr Hague called for the "unacceptable" restrictions on access to Gaza to be lifted and said there was a "clear need for Israel to act with restraint and in line with international obligations".
As urgent talks were called at the United Nations and the European Union, hundreds of protesters gathered outside Downing Street in a noisy demonstration against the Israelis' actions.
They blocked Whitehall shouting "Free Palestine" and carried flags and banners with slogans such as "Stop Israel's War Crimes in Gaza" and "End the Criminal Siege of Gaza".