The prime minister has defended the strong arm tactics of the Italian authorities ahead of the G8 summit in Genoa.

As the Italian town comes under "siege", Downing Street blamed an "anarchist travelling circus" intent on hijacking legitimate protests for the ring of steel which has been thrown up around world leaders.

"What is absolutely clear - you saw it at Seattle and Gothenberg - is that people hijack events for their own ends to create mayhem," the prime minister's spokesman said on the eve of the summit.

Following a series of violent protests at international summits, Tony Blair told the Cabinet on Thursday that it remained important that "we keep making the arguments" for institutions such as the G8 and the World Trade Organisation.

He said it was vital that world leaders continue to stress the good work they were doing in areas such is liberalising world trade and in developing programmes to combat diseases such as Aids in the third world.

Downing Street contrasted legitimate protests, such as the effective Drop the Debt campaign run by Jubilee 2000, with the actions of a "mindless yob element" which had forced summit organisers to resort to "extreme precautions".

The prime minister's remarks came amid criticism of the tactics used by the Italian authorities in Genoa.

Legitimate protesters claim they have been refused entry to the country and forced to return to the UK as a result of a "heavy-handed" approach.