Police cannot rule out pre-emptive strikes against anarchists plotting to disrupt the royal wedding, Scotland Yard has said.

In one of the biggest security operations in the history of the Metropolitan police, just under 5,000 police officers - including armed and undercover teams - will be on duty on 29 April in the city of Westminster and around the centre of London.

So far, two groups have indicated that they wish to protest: Muslims against Crusades, who asked to demonstrate outside Westminster Abbey but were refused permission, and the English Defence League. The EDL indicated it would mount a demonstration if Muslims against Crusades did so.

Sixty individuals who have been arrested at past demonstrations, such as the TUC anti-cuts protest and the student demonstration against the introduction of fees, have been banned from the city of Westminster as part of their bail conditions. Action will be taken against them if they enter the city on the day.

In addition, the Met is working with forces across the country and is using "spotters" to identify those within the so-called "black bloc" of anarchists intent on causing trouble.

Should evidence emerge that groups are planning to commit criminal acts, pre-emptive action will be taken, a Scotland Yard spokesman said. This could range from breaking up a squat where individuals are gathered, under breach of the peace legislation, or moving in to break up and arrest individuals if evidence suggests they are conspiring to commit criminal acts.

"It is very difficult to build an intelligence picture, but we cannot rule out pre-emptive action," said the spokesman.

Assistant commissioner Lynne Owens, who is head of public order at Scotland Yard, said: "If anyone comes to London on the day of the royal wedding intending to commit criminal acts, we will act quickly, robustly and decisively so that it is a safe and happy environment for everyone else, who wishes to be here and celebrate."

The Met is also getting intelligence from the Fixated Threat Assessment Centre, a police unit set up in 2006 together with mental health agencies to identify individuals who are obsessed with members of the royal family, politicians or celebrities. Details of a handful of people are understood to have been passed on to police officers.

Under the Serious Organised Crime Act 2005, the area around the houses of parliament and Westminster Abbey is designated an exclusion zone where unauthorised demonstrations are not allowed.

An early conversation between Owens and Muslims against Crusades has been held. The refusal of permission to demonstrate outside the abbey does not prevent them from protesting elsewhere, but Owens warned that any action to burn the union flag would be seen as an offence under the Public Order Act.

Owens said there was no intelligence to suggest a terrorist threat to the wedding, so police were not planning at this stage to use stop-and-search powers under the new section 47a of the Terrorism Act. This section replaces section 44, which was found to be illegal under European law.

She said it was likely that searches would be carried out under the two other powers available to police: section one of the Police and Criminal Evidence Act and Section 60 of the Criminal Justice Act 1996. The latter allows police to designate an area a section 60 zone, in which officers are able to stop and search individuals without requiring evidence of wrongdoing.

A decision on how wide the section 60 area would be is likely to be made nearer the day of the wedding.

The police will be protecting 80 dignitaries, as well as the expected tens of thousands of visitors from all over the world.

A mobile unit of officers will be used to deal with any violence or unruly demonstrations outside the exclusion zone area, which covers Westminster Abbey and the route to be taken by Prince William and Kate Middleton.