A huge expat crowd has attended the Anzac dawn service in London.

More than five thousand New Zealanders and Australians packed out the war memorial at Hyde Park.

Sam Heveldt, who's originally from Christchurch, says security was also heightened.

He says metal detectors and scanners marked the gate to get in.

"The queue actually stretched right around Hyde Park corner and looped back three times. There were a lot of people there."