Lancashire police said on Saturday that the seizure was made as part of an anti-terror raid in the northwest of England.
The convoy had been organised by the pro-Palestinian organisation Viva Palestina and left central London on Saturday, the group's website said.
It said the vehicles, which include a fire engine and ambulances, were carrying clothes, blankets and children's toys.
It said volunteers planned to drive 8,000km through France, Spain, Morocco, Algeria, Tunisia, Libya and Egypt where they had hoped to cross the border at Rafah into Gaza on March 2.
Lancashire police said the cargo from the three seized vehicles was being "thoroughly checked by search officers who are working as quickly as they can to ensure that the parcels can be released and continue on to their destination".
Lancashire police said they were searching five houses in the town of Burnley on Saturday after arresting nine men under anti-terrorism laws on a motorway near Preston on Friday.
Six of the men have been released and the remaining three are still in custody. Police can hold them without charge until Sunday night.