Nine people are being questioned by police following extensive damage at an arms factory where protesters claim military components are being made for Israeli warplanes bombing Gaza.

The group, which calls itself Smash EDO, entered the EDO MBM Technology plant in Moulsecoomb, Brighton, in the early hours of this morning. During the incident computers and furniture were hurled from the windows of the Sussex factory. Police described the damaged as "substantial".

Demonstrators said they were "decommissioning" the site in protest against the killings of Palestinians in the Gaza Strip by the Israeli military. They said equipment made at the plant was being used in Gaza by the Israeli air force.

DCI Graham Pratt of Sussex police said: "Windows had been smashed and offices turned over in what I would describe as wanton vandalism, but with machinery and equipment so targeted that it could have been done with a view of bringing business to a standstill. The damage is significant and the value substantial."

EDO MBM is the sole British subsidiary of US weapons company EDO Corp. From its Moulescoomb base it manufactures laser-guided missiles that have been used extensively in Iraq, the Palestinian territories and Somalia.

The weapons were reportedly used by Israel against Lebanon in 2005, and have also been allegedly used in the occupied Palestinian territories.

Thousands of people gathered this afternoon for demonstrations across the country in a third weekend of protests against the Israeli attacks on Gaza.

Organisers of a rally in Birmingham said more than 5,000 people had turned up in the city centre. In London, the former Labour cabinet minister Tony Benn was among the speakers due to address crowds in Trafalgar Square.

He said: "It is a moral responsibility for all of us. People are being killed not so far away from here, women and children."