The Metropolitan Police were accused in the High Court of "suppressing anti-monarchist sentiment" during the Royal Wedding in London last year.
The accusation came as 20 individuals among scores arrested or searched last April asked the court to rule the Commissioner of Police operated an unlawful policy that violated the democratic right to protest.
Human rights activists say the case has implications for the policing of other major events, including the Diamond Jubilee.
Lord Justice Richards, who is hearing the judicial review application with Mr Justice Openshaw, indicated that the court's ruling was likely to come too late to influence the Jubilee.
Karon Monaghan, QC, appearing for 15 applicants, said her clients were all pre-emptively arrested on April 29, 2011, as Prince William married Kate Middleton.
Their arrests occurred in four locations in London, when police suspected them of being about to commit breaches of the peace.
Ms Monaghan told the court the case touched upon "the most important of constitutional rights, namely the rights to free expression and to protest".
The Met denies the existence of an unlawful policy of arrest and detention.
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