Police are coming under political pressure to explain why no arrests were made during demonstrations in which protesters chanted threatening slogans.
Many Muslims do not believe the protesters are representative
The protests on Friday in London over cartoons of the Prophet Muhammad saw slogans and placards glorifying the 7 July London bombings.
Shadow Attorney General Dominic Grieve questioned why no-one was arrested.
Police said they were studying footage of the protests but refused to say if any prosecutions would go ahead.
On Saturday more protesters, organised by the Hizb ut-Tahrir group, gathered outside the Danish embassy in London. This demonstration passed off peacefully.
Among the images which have sparked outcry among Muslims is one of Muhammad with a bomb-shaped turban on his head. Newspapers in Spain, Italy, Germany and France reprinted the material.
A Met Police spokeswoman said: "We have stated that arrests if necessary will be made at the most appropriate time.
"This should not be seen as a sign of a lack of activity of the Met Police.
"Specialist officers were deployed on both days' demonstration to record any potential event should it be needed at any point in the future."
Mr Grieve said: "Certainly I'm concerned if there is any delay in bringing them to justice. What took place was outrageous.
"It's a clear breach of the criminal law - the existing criminal law, not new laws about incitement.
"To go out on the street and call for the killing of others is something which is intolerable.
Some of the protesters glorified Bin Laden and terrorist atrocities
"Now the police may well have had good operational reasons not to jump in and arrest people on the spot.
"But I would be very concerned if one was to learn that in fact it's now proving impossible to detect or identify those who perpetrated this because arrests were not made at the time."
Cabinet Minister Peter Hain took a more cautious line on prosecutions, warning that politicians must not try to "second guess" police.
Speaking to Sky News he said: "If people are on our streets inciting terrorism or promoting suicide bombings or anything like that, then they should be dealt with and dealt with toughly and firmly and they will be.
"But that is the police's responsibility and they will discharge that."
Meanwhile, Inayat Bunglawala, a spokesman for the Muslim Council of Britain, said that Muslims would be in favour of arresting those who waved offensive placards or banners.
He told Channel 4 News: "I think the police were right to have taken footage of the event and identified the ringleaders, because although several hundred people were there, the actual placards were being held by a tiny group of extremists."
He added: "Those extremists who were inciting violence were trying to hijack genuine feelings amongst Muslims for a more violent agenda.
"There will be no sympathy for them when they are charged by the police."
The Labour MP Shahid Malik, who is on the Home Affairs Select Committee, wrote a letter to Sir Ian Blair, head of the Met Police, on Friday calling for prosecutions.
He said he had every confidence suitable action would be taken.
Mr Malik added: "Police are professionals at dealing with crowd and disorder matters and I think they're best placed to make those kinds of judgements.
"I believe that prosecutions should follow. No matter how much offence cartoons may or may not cause, it can never justify violence."
The cautious tone was echoed by shadow foreign secretary William Hague, who said it was up to the police to decide what to do in individual cases.
But he added: "It is very concerning that, in a very small number of cases, people have called for terrorist acts and have, in effect, done what amounts to incitement to murder."
Banners waved during a demonstration in London on Friday featured slogans including "Europe, your 9/11 will come" and - in an apparent reference to the four July 7 suicide bombers - "Europe you will pay, fantastic 4 are on their way".