The man, who is said to be from Britain’s Special Forces but cannot be identified for security reasons, had been drinking a coffee at the Westgate mall in Nairobi when militants stormed it on Saturday.
He reportedly went back and forth between the building and the outside world around 12 times, helping people escape from the complex.
One picture showed him helping two women out, a gun tucked into his waistband.
A friend in Nairobi told the Daily Mail: “What he did was so heroic. He was having coffee with friends when it happened.
“He went back in 12 times and saved 100 people. Imagine going back in when you knew what was going on inside.”
On-duty Special Air Service soldiers were also at the scene last night, helping the Kenyan authorities in their final attempt to free the hostages.
A team of the soldiers was advising the local military as they stormed the shopping centre, and were on standby to take part.
Amina Mohamed, Kenya’s foreign minister, said today that a British woman extremist and “two or three” Americans were among the terrorists behind the attack.
“From the information that we have, two or three Americans (were involved) and I think, so far, I have heard of one Brit… a woman … and I think she has done this many times before,” she told the US’s PBS NewsHour programme.
The Kenyan government needs to work “much more with the US and the UK because both the victims and the perpetrators came from Kenya, the United Kingdom and the United States”, she added.
David Cameron, the Prime Minister, who returned from a Balmoral break with the Queen to chair a meeting of the emergency Cobra committee in Whitehall yesterday, said it appeared at least six Britons had died in the incident.
Philip Hammond, the Defence Secretary, warned the death toll could be even higher.
“It is possible we will discover further British nationals once the building is fully secure,” he said.