A 50-year-old man, held on suspicion of conspiracy to murder, becomes the tenth person arrested over the hacking to death of 25-year-old Lee Rigby near a barracks in Woolwich on Wednesday.
This includes the two prime suspects, Michael Adebolajo, 28, and Michael Adebowale, 22, who remain under armed guard in separate London hospitals after being shot by police at the murder scene.
The attack by two men spouting Islamist rhetoric has sparked community tensions, with several mosques attacked in recent days and a charity reporting a surge in anti-Muslim incidents.
EDL leader Tommy Robinson addressed the largely male crowd on Whitehall, many of them shouting "Muslim killers off our streets", saying: "They've had their Arab Spring. This is time for the English spring."
The crowd, about half the number who attended a protest in the northern city of Newcastle on Saturday, shouted "coward" as Robinson told them the prime minister was on holiday "because he doesn't care".
Cameron, who is spending a week in Ibiza with his family, last week joined calls by Muslim and Christian leaders for calm, saying: "We will defeat violent extremism by standing together.