British counter-terrorism police were Thursday investigating the murder of a man thought to be a soldier, who was butchered and beheaded on a busy London street by two men shouting Islamist slogans.
The attackers, wielding knives including a meat cleaver, carried out the attack a few hundred metres (yards) from the Royal Artillery Barracks in Woolwich, southeast London, then delivered an Islamist tirade to passers-by .
Prime Minister David Cameron  condemned the "appalling crime", adding: "There are strong indications that it is a terrorist incident."
He cut short a visit to Paris to fly back for a meeting of the government's emergency response committee, COBRA, which had already met in the hours following the attack.
Media reports citing witnesses said the men first ran over their victim in a car before finishing him off with the knives.
Eyewitnesses described how after the two men carried out their attack, they stayed at the scene, asking passers-by to photograph and film them.
Armed police shot and wounded the two attackers after they ran at officers, said witnesses. Both were being treated in hospital, said police.
Amateur footage of one of the men carrying a blood-stained knife and meat cleaver shows him saying: "We swear by almighty Allah we will never stop fighting you."
The man, a black man dressed in a hooded jacket and black woolly hat, speaks in a London accent.
"We must fight them as they fight us. An eye for an eye, a tooth for a tooth," he says.
"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."
Reports said the man attacked was wearing a t-shirt bearing the logo of the British military charity Help for Heroes.
Several eyewitnesses said he had been decapitated.
Rapper Boya Dee, who witnessed the incident, wrote on his Twitter account: "Ohhhhh myyyy God!!!! I just see a man with his head chopped off right in front of my eyes!"
Ingrid Loyau-Kennett, 48, told the Daily Telegraph that when she asked the assailants why they had carried out the attack, one of them told her: "We want to start a war in London tonight".
Around 250 members of the anti-Islamist English Defence League  were involved in minor scuffles with police at Woolwich Arsenal train station, near the scene of the attack, before dispersing.
Police later arrested two people suspected of trying to carry out reprisals.
Officers arrested a 43-year-old man who was detained in Braintree, southeast England, after reportedly walking into a mosque with a knife.
They detained another man in nearby Gillingham on suspicion of racially aggravated criminal damage to a mosque, police said.
Cameron, speaking to journalists before flying back from Paris, called the attack "truly shocking".
Speaking at a joint news conference with French President Francois Hollande, he added: "We've had this sort of attack before in our country and we never buckle in the face of them.
"People across Britain, people in every community, I believe, will utterly condemn this attack."
Hollande, expressed solidarity with Britain, at "the cowardly killing of a British soldier", but the British prime minister did not publicly confirm that the victim was a soldier.
In London, a local member of parliament, Nick Raynsford, said he understood the dead man had been a soldier. Security was reinforced at all army barracks in London following the incident.
The Muslim Council of Britain  said the killers' use of "Islamic slogans" indicated they were motivated by their faith.
A statement from the council said: "This is a truly barbaric act that has no basis in Islam and we condemn this unreservedly. Our thoughts are with the victim and his family."
Police were called at 2:20 pm (1320 GMT) to reports of one man being assaulted by two others.
Britain's top policeman, Metropolitan Police Commissioner Bernard Hogan-Howe confirmed two men had been arrested following the "shocking and horrific" attack.
"We have launched a murder investigation, being led by the Counter Terrorism Command," he added.
Police had met local community representatives and extra officers were patrolling the area, he said. Appealing for calm, he added: "The shock that we all feel at what has happened must bind us together."
The United States condemned the attack.
"We stand with our UK allies in the face of such senseless violence," State Department spokesman Patrick Ventrell said in a statement.