British embassy officials are trying to establish if any UK citizens were caught up in a terrorist bombing which ripped through a cafe in a popular Moroccan tourist spot, killing at least 15 people.
Foreign Secretary William Hague sent his condolences to the families of the victims, mostly foreign visitors.
There was no official indication from the Moroccan authorities to verify a media report that one Briton was among the dead in the worst attack there in eight years.
One British national was said by the casualty unit chief at Marrakech's main Tofail Hospital to be among dozens injured by the blast.
Gas canisters were initially blamed for the massive explosion in the main Djemma el-Fna square - which is well-known for its snake charmers, fire breathers and old town. Within a short time, however, a Moroccan government spokesman said it had been a terrorist attack but that it was too soon to lay the blame on any particular group.
British holidaymakers described how the front of the cafe had been blown away during the busy lunchtime period and the first floor left "in ruins".
Mr Hague said he was "shocked and saddened" at Thursday's "deeply worrying" explosion.
He said: "I offer my condolences to all those who have lost relatives or been injured. Initial reports that this may have been a result of terrorism are deeply worrying. All acts of terrorism are utterly reprehensible. British officials are in contact with the Moroccan authorities to establish the facts and to provide consular support to any British nationals who may have been caught up in the blast."
Andy Birnie, of north London, who is on his honeymoon, witnessed the blast in the square He said: "There was a huge bang, and lots of smoke went up. There was debris raining down from the sky. Hundreds of people were running in panic, some towards the cafe, some away from the square. The whole front of the cafe is blown away."
Briton Hugo Somersham-Jones, who lives near the square, told the BBC: "It sounded like a bomb. I went outside and saw smoke and got to the cafe and saw falling masonry. I came out to the main square and saw the first floor of the cafe in ruins."