LONDON - Prime Minister David Cameron vowed on Thursday that Britain would be resolute against violent extremism following the gruesome murder of a soldier by two suspected Islamists on a London street.

After chairing a meeting of security chiefs the day after the soldier was hacked to death in broad daylight, he said Britain's communities would unite in condemning an attack he described as a "betrayal of Islam". "This country will be absolutely resolute in its stand against violent extremism and terror," he told reporters outside his Downing Street office.

"This was not just an attack on Britain and on the British way of life. It was also a betrayal of Islam and of the Muslim communities who give so much to our country."

Wielding knives including a meat cleaver, two men carried out the attack on Wednesday afternoon near the Royal Artillery Barracks in Woolwich, southeast London, then delivered an Islamist tirade to passers-by. Chilling amateur footage of one suspect showed him still holding blood-stained knives, telling a member of the public they killed the victim "because Muslims are dying daily by British soldiers".

The suspects were shot by police after the attack and spent the night in separate hospitals under armed guard.

The victim has yet to be formally identified. His murder is the first fatal Islamist attack in Britain since 2005, when suicide bombers killed 52 people on London's transport network.

Cameron cut short a visit to Paris in order to chair a meeting of the government's emergency response committee, COBRA, which had already met in the hours following the attack. Defence and interior ministers attended the meeting along with intelligence chiefs, the head of Scotland Yard and London mayor Boris Johnson.

Meanwhile counter-terrorism police have launched a large-scale investigation into the killing. Officers were conducting searches near the murder scene on Thursday, and confirmed that they had raided an address in Lincolnshire, eastern England. Police investigating the brutal murder of a soldier by suspected Islamists in London said on Thursday they had arrested two alleged conspirators.

A man and a woman, both 29, were arrested on suspicion of conspiracy to murder and are in custody at a police station in the south of the capital, a Scotland Yard statement said. Two suspects aged 22 and 28 who were shot by police at the murder scene in Woolwich, southeast London, on Wednesday, remain in separate London hospitals. They were both "in a stable condition. Their injuries are not life threatening", police added.

The arrests came after police searched six residential addresses: three in south London, one in east London, one in north London and one in the village of Saxilby in eastern England. "This is a large, complex and fast-moving investigation which continues to develop. Many lines of inquiry are being followed by detectives and the investigation is progressing well," the police statement said. "Officers have been gathering information from witnesses, social media and a painstaking trawl of CCTV footage in the area is taking place.

"Forensic experts and specialist search officers have been carrying out a detailed examination of the scene in Woolwich. A number of items have been recovered from the scene." Media reports said the suspects were believed to be Britons of Nigerian descent, but they were not thought to have links to Nigerian Islamist groups such as Boko Haram.

Cameron declined to comments on reports that the men were previously known to intelligence officials, but he hinted that police and security agencies could face probes into how the attackers slipped under the radar.

In the video, which has gone viral on the Internet, one of the suspects is seen walking around with bloody hands and tells the camera: "You people will never be safe."

Speaking in a London accent the man, who is black, makes various political statements including a demand for Cameron to "bring our troops back".

Shocked eyewitnesses described how the men stayed at the scene after the killing, asking passers-by to photograph and film them.

"I apologise that women have had to witness this today, but in our lands our women have to see the same," said the suspect, who was wearing a hooded jacket and a black woolly hat.

Media reports citing witnesses said the men first ran over their victim in a car before finishing him off with the knives.