Getty ImagesThe number of arrests at Saturday's demonstration in London in support of banned group Palestine Action has risen to 532.The Metropolitan Police said the majority of arrests - 521 - were for displaying placards in support of Palestine Action at Westminster's Parliament Square, and one at a Palestine Coalition march.Six arrests were for assaulting police officers, two for breaching Public Order Act conditions, one for obstructing a constable and one for a racially aggravated public order offence.It was the biggest protest since the government proscribed the group in July under the Terrorism Act, making membership of or support for it a criminal offence, punishable by up to 14 years in prison.The force previously said that the number of arrests was the largest made by the force on a single day in the last 10 years.On Sunday the Metropolitan Police said the average age of those arrested was 54, and the most arrests - 147 of them - were of people aged between 60 and 69.It said its counter terrorism team was now working on bringing charges against those accused of supporting Palestine Action.The force said: "Over the coming days and weeks, officers from the Met's Counter Terrorism Command will work to put together the case files required to secure charges against those arrested as part of this operation."Protesters whose details could be confirmed at temporary prisoner processing points nearby were released on bail to appear at a police station at a future date, under condition not to attend future demonstrations related to Palestine Action.But 212 protesters who refused to provide their details or were found to have been arrested while already on bail were taken into custody.Police said as of 13:00 BST on Sunday, 18 of them remained in custody but were expected to be released during the day.Security expert Lizzie Dearden told the BBC that the laws the protesters were arrested under "were not designed for a group of this nature or a group of this size"."When [the Terrorism Act] was drafted in the late Nineties, the kind of terrorist group that was really being envisaged were groups like the IRA, al-Qaeda, armed militant groups that were operating often abroad."So the law is being applied in a completely new way."Home Secretary Yvette Cooper thanked police for their response following the demonstration, and said the banning of Palestine Action was based on "strong security advice following serious attacks the group has committed" as well as "plans and ideas for further attacks"."Many people may not yet know the reality of this organisation, but the assessments are very clear - this is not a non-violent organisation," she said.But charity Amnesty International's chief executive Sacha Deshmukh described the mass arrests as "deeply concerning".He said: "We have long criticised UK terrorism law for being excessively broad and vaguely worded and a threat to freedom of expression. These arrests demonstrate that our concerns were justified."