He gave no indication what that land was.
"We must fight them as they fight us," the man told the camera as people milled around behind him. The camera then panned away to show a body behind the man.
The Associated Press examined the footage to verify its authenticity. The AP cross-referenced images from the scene, aerial shots, the location of a car behind the alleged attacker and appearance of a body and car in the background of the image.
The British Cabinet's emergency committee immediately called a meeting and the prime minister's office said security was stepped up at barracks across London. Cameron cut short his Paris trip to return to London and his office said he would chair another emergency committee meeting Thursday.
The barracks - which house a number of the King's Troop Royal Horse Artillery and independent companies of the Grenadier and Coldstream Guards - were the site of shooting events during the 2012 London Olympics.
Fred Oyat, a 44-year-old who lives in a high-rise near where the attack occurred, said he heard four gun shots and then went straight to the window.
"I saw one man lying there bleeding, another lying on the pavement being disarmed. A policeman was pointing a gun at him. A third man was lying further up the street . . . he was bleeding profusely," Oyat said. "There were four knives on the ground - big kitchen knives. The knives were very bloody."
David Dixon, head teacher of a nearby primary school, saw a body lying in the road outside and said police told him there was a serious incident. He told the BBC he then made sure students were inside and put the school into a lockdown mode. He said he then heard shots fired.
The Independent Police Complaints Commission, which is called in when officers are involved in shootings, confirmed that it is investigating the attack.