At least 39 people have been killed and 150 wounded in a terrorist attack in a shopping mall in the Kenyan capital Nairobi.
Several children were among those massacred by al Qaeda-linked terrorists armed with guns and grenades.
Some of those killed were executed after failing to recite a Muslim prayer at gunpoint.
Foreign Secretary William Hague said British nationals were undoubtedly caught up in the "callous and cowardly and brutal" attack at the upmarket mall.
It is understood there may be dozens of hostages, possibly including Britons, still trapped in the mall.
Gunmen wearing bullet-proof vests opened fire and detonated grenades inside the Westgate centre, popular with expatriates and wealthy Kenyans.
Some people were shot at the entrance to the mall as they tried to escape, picked off from rooftops.
One image showed a dead child cuddled up to his dead mother inside a car. Other pictures showed a bloodied woman being wheeled out of the mall in a shopping trolley.
Mothers caught up in the attack were pictured running to safety clutching small children.
Police chief Benson Kibue described the incident as a terrorist attack and said that up to 10 men exchanged gunfire with police.
The Somalia-based al Qaeda-linked al Shabaab terror group has claimed responsibility for the attack and warned of more attacks.
Police sources said one gunman had died from bullet wounds.
Al Shabaab militants have reportedly said there will be "no negotiaions" over the siege, which continues.
It is understood two French citizens have been killed in the attack and Americans are also thought to be involved.
Saadia Ahmed, a radio presenter from Nairobi caught up in the attack, told Sky News how the attack began.
"We heard three explosions outside the building then all of a sudden we heard gunshots and people ducked down," she said.
"A lot of people were shot while they were trying to escape. I saw one of the gunmen with an AK-47 and later two of them were talking and it sounded like Somali or Arabic."
Ms Ahmed said the attackers released people who were able to prove they were Muslim.
"I witnessed a few people get up and say something in Arabic and the gunmen let them go. A colleague of mine said he was Muslim and recited something in Arabic and they let him go as well.
"I saw a lot of children and elderly people being shot dead. I don’t understand why you would shoot a five-year-old child. They were firing at random at anyone who tried to escape."
Earlier, Abbas Guled, a spokesman for the Kenyan Red Cross, said: "The casualties are many and that's only what we have on the outside. Inside there are even more casualties and shooting is still going on."
According to Nairobi-based journalist Abdi Osman Adan, witnesses reported seeing the attackers "firing at any police officers who tried to approach the building".
Security forces are still trying to rescue those trapped inside the centre hours after the start of the siege, going from shop to shop to evacuate shoppers and staff.
Satpal Singh, who was in a cafe when the attackers struck, said he ran downstairs before he was shot near the exit.
"A Somali guy shot at me," he said. "He was carrying a rifle, an AK-47."
A witness, who gave only his first name, Jay, said: "They were not speaking Swahili. They spoke something that seemed like Arabic or Somali."
In a message posted on Twitter , Kenya Police urged the public "to remain calm" and not to speculate about the attack.
"We urge you to stay away (from the centre)," they added.
Westgate is situated in western Nairobi.
According to the centre's website, it is the city's "premier shopping mall" and offers a "serene and safe environment away from the city centre hubbub".
A Downing Street spokesman said: "The Prime Minister spoke to the President of Kenya, Uhuru Kenyatta, late this afternoon about the attack at Westgate shopping centre in Nairobi."