Amid tight security Europe's biggest arms fair was opened by the defence secretary in London on Tuesday.

An estimated 54 arrests have been made in the run-up to the event.

Police have targeted protestors who argue it will attract countries with poor human rights records.

Buyers are expected to come from countries including Algeria, Angola, Colombia, Pakistan and India.

Around half the exhibitors are British, while a further fifth are from the United States, and 30 per cent from other NATO countries.

The policing bill for the fair is expected to top £1 million.

"Our concern would be if the demonstration turns into riots or damage and we have got to be prepared to deal with that," said Andy Trotter, deputy assistant commissioner at the Metropolitan Police.

However, the police have been criticised by civil rights groups for allegedly using anti-terrorism laws against protestors.

"To use it as they are using it against protesters at an arms fair seems to us to be blatantly illegal," said Liberty spokesman Barry Hugill.

"The only reason to use it is to break up the demonstration, to stop people, to take away their basic right to freedom of speech."

However, a police spokesman said that stop-and-search methods were being used under the Police and Criminal Evidence Act, not anti-terror laws.

The Campaign Against the Arms Trade group has pledged to disrupt the event, but has insisted that all action by the group will be non-violent.

"This arms fair is a bringing together of companies and countries with terrible track records," said a spokesman.

"First and foremost we hope we will get it cancelled, or create a public outcry big enough to make sure it doesn't happen again."

However, a spokesman for organisers Defence Systems and Equipment International, defended the event.

"This is an exhibition, mainly for contractors and sub-contractors," he said.

"It's an industry-to-industry show."

Exhibitors have signed forms pledging not to bring illegal weapons systems, which are then monitored by the Ministry of Defence.

"We have strict export licence criteria and going to the exhibition in no way implies that you can buy the equipment," he added.

"Defence brings £5 billion a year to the UK and benefits between 70,000 and 100,000 jobs."