Counter-terrorism officials are trying to determine if the machete killing of a man in London by two men was a random act or part of a larger operation.
British Prime Minister David Cameron called an emergency meeting of the government's crisis committee after the man was hacked to death outside a military barracks by two men shouting Islamist slogans on Wednesday.
The pair then waited for police, who shot and wounded them before arresting them after the daylight attack in the south-east district of Woolwich about 2pm (local time).
The BBC reports Whitehall sources have confirmed that the victim was a member of the armed forces.
Footage has emerged showing one of the attackers wielding a bloodied knife and meat cleaver. He makes a series of political statements before walking back to towards the victim lying prone in John Wilson Street. He said he carried out the attack because British soldiers kill Muslims every day.
"We must fight them as they fight us. An eye for an eye, a tooth for a tooth," he said to the camera, adding: "I apologise that women have had to witness this today, but in our land our women have to see the same. You people will never be safe. Remove your government, they don't care about you."
Witnesses said they shouted "God is greatest" in Arabic while stabbing the victim and trying to behead him.
The Prime Minister cut short a visit to France because of the incident, which officials are treating as a suspected terrorist attack. David Cameron said Britain would not be cowed by the event.
"It is the most appalling crime. We obviously are urgently seeking and the police are urgently seeking the full facts about this case. But there are strong indications that it is a terrorist incident. We've had these sorts of attacks before in our country - and we never buckle in the face of them."
Home Secretary Theresa May called a rare meeting of the government's emergency security committee over the "sickening and barbaric" attack.
"The attack was an attack on everyone in the United Kingdom and it will be condemned by people from every community. Our thoughts and prayers are with the victim and his family. Despicable acts like these will not go unpunished."
Metropolitan Police Commissioner Sir Bernard Hogan-Howe appealed for calm after a crowd of 100 - including members of the far right English Defence League - later took to the streets in Woolwich in protest but were contained by riot police. Two mosques were also attacked.
Security has been increased at army barracks across London.
The attack, just a month after the bomb attacks on the Boston Marathon, has revived fears of so-called "lone wolves" who might have had no direct contact with al Qaeda. Sources familiar with the investigation say authorities are investigating a possible link to Nigeria, Reuters reports.
A witness, identified only as James, said two men had attacked another man, aged about 20, who was wearing a T-shirt of military charity Help for Heroes, the BBC reports.
He told a local radio station the men dragged the victim from the footpath and dumped his body in the middle of the road.
James said after the "horrendous" attack, the two men, also in their 20s, stood around, waving knives and a gun and asked people to take pictures of them.
Police said when they arrived at John Wilson Street in Woolwich they found one man who was later pronounced dead.
Officers shot the two armed men and both are being treated for injuries in separate London hospitals.