A London-wide ban on Extinction Rebellion protests was "wholly uncertain, an abuse of power and irrational", High Court judges have heard.
Metropolitan Police imposed a blanket ban on the climate change protests on October 14, as demonstrations in the capital entered their second week.
But at a hearing in London today, lawyers for the activist group argued that the city-wide ban, made under Section 14 of the Public Order Act, was unlawful because the Met went beyond its powers by prohibiting "multiple assemblies, both ongoing and intended".
Phillippa Kaufmann QC said: "Insofar as it applied to existing assemblies, because these were not clearly identified - even by geographical location - it was impossible for any individual who wished to protest to know which protest they could join and which they could not.
"The condition clearly failed to distinguish between assemblies and protesters participating in assemblies."
Those bringing the action include Green Party politicians Baroness Jenny Jones and Caroline Lucas MP, Labour MP Clive Lewis and campaigner and journalist George Monbiot - who was arrested after the ban came into force.
The Met contends the ban, which is no longer in place, was lawful and was the only way of tackling the disruption caused by the group.
Lawyers for the force said it was "clear and undeniable" that XR regarded the Autumn Uprising as a single protest and that it was an "established tactic" of the group to "move around and change locations to frustrate law enforcement".
The force said 1,832 people were arrested during the protests, and more than 150 were charged with offences.
Lord Justice Dingemans and Mr Justice Chamberlain are expected to give their ruling on the case at a later date.