At least five people were arrested after protests at a factory which pro-Palestinian activists claim supplies weapons to Israel.
A small number of demonstrators from group Palestine Action entered the Elbit Ferranti factory in Oldham, Greater Manchester, on Tuesday morning.
Activists sprayed computers in red paint, broke windows, chained themselves to the premises and stuck "war crime scene" barrier tape across the front doors.
Five people were arrested - two on suspicion of criminal damage and three on suspicion of burglary - after being removed from the roof, Greater Manchester Police said.
The group also took similar action at the UK headquarters of Israeli firm Elbit Systems in a business complex near Bristol.
The plants have been repeatedly targeted by activists who claim the business is involved in the supply of weapons used by the Israeli military against Palestinians.
Demonstrators have pledged to blockade the sites for "as long as possible".
In a statement, Palestine Action said: "Elbit has absolutely no place in any civilised society and their existence is an utter stain on the UK."
A Greater Manchester Police spokesman said: "Following an incident which began this morning at around 6am at a business premises on Greenacres Road, Oldham, police can confirm that a total of five people have been arrested.
"Two women have been arrested on suspicion of criminal damage and a further three men have been arrested on suspicion of burglary after being removed from the roof of the premises."
Avon and Somerset Police said it had been monitoring an incident involving a "small number of people".
The protests come after the Ministry of Defence confirmed earlier this year that Elbit had won a £102 million contract for new "sensor-to-shooter" surveillance system, which feeds real-time battlefield targeting information to infantry, aircraft and artillery.