A man and a woman, both 29, were arrested on suspicion of conspiracy to murder in relation to the Woolwich terror murder of the soldier identified as Drummer Lee Rigby, Scotland Yard said.
A total of four people, including the two men shot by police have now been arrested in connection with the killing - and the latest two arrested were being questioned at a south London police station.
Scotland Yard said six residential addresses were searched on Thursday as part of the investigation - three in south London, one in east London, one in north London and one in Lincolnshire.
After chairing an emergency meeting of the high-level COBRA crisis group, Cameron said Britain's communities would unite in condemning Wednesday's attack which he described as a "betrayal of Islam".
Cameron, who cut short his visit to Paris, said the terror attack was "deeply shocking" that has "sickened us all".
"What happened yesterday in Woolwich has sickened us all. We have lost a brave soldier," Cameron said, addressing the media outside 10 Downing Street.
"It was an attack on Britain and it was also a betrayal of Islam. There is nothing in Islam that justifies this truly dreadful act," he said.
"This country will be absolutely resolute in its stand against violent extremism and terror. We will never give in to terror or terrorism - in any of its forms...Confronting extremism is a job for us all," he added.
Anti-terror police forced their way into a flat and shattered the door frame as they raided an address believed to belong to one of the attackers on a Greenwich housing estate.
They also raided an address in Saxilby, Lincolnshire, in connection with the attack.
"We can confirm that the Metropolitan Police executed a search warrant under PACE (Police and Criminal Evidence Act) at an address in Lincolnshire," police said.
"This is in connection with the ongoing investigation into the murder of a man in Woolwich. The Metropolitan Police are not prepared to discuss the matter further at this stage," they said.
Both suspects in the killing were known to security services, senior Whitehall sources have said.
One of the suspects is understood to be 28-year-old Muslim convert Michael Adebolajo, who is believed to be a British citizen of Nigerian descent, according to sources.
He and another man were under arrest in hospital after being shot by police following Wednesday's attack in Woolwich. Shortly after the killing a man, thought to be Adebolajo, was filmed by a passer-by, saying he carried out the attack because British soldiers killed Muslims every day.