There have been a number of arrests as mass protests began against the G20 summit in London. Police launched one of Britain’s biggest-ever security operations as thousands of anti-globalisation and anti-war demonstrators swamped the centre of the city. The protests have been mostly good-natured, but two people were apprehended for allegedly carrying knives and another suspected of threatening a police officer.The security operation has stretched resources to the limit. All police leave has been cancelled, and much of the centre of the city is sealed off. Most marchers insisted the protests would be peaceful, but there are fears of violence later. More demonstrations are planned for tomorrow, when G20 world leaders open their economic summit.