"We have learnt lessons from Seattle, Genoa and Davos. The same will not be allowed to happen," Sean Tshabalala, director of VIP protection for the South African police, told reporters.

He said activists planning pickets or marches must apply for permits in advance, which would be granted if demonstrators keep to assigned areas and follow an agreed code of behaviour.

"Spontaneous activities are just not acceptable. A group of people gathering spontaneously will be picked up and locked up," he told reporters.

Demonstrators have hit virtually every international political gathering in recent years to protest against capitalism, globalisation, or military action.

They were notable in their 1999 success in derailing the launch of a new round of World Trade Organisation negotiations in Seattle.

The World Summit on Sustainable Development (WSSD), a follow-up to the 1992 Rio Earth Summit, was brought forward to August 26 to September 4 from its previous dates for fear that the original period coincided with the anniversary of the attacks on the United States.

"We are prepared for everything - airborne, sniper or mortar attacks, violent illegal protests, kidnappings, lunatic attacks or biological attacks like with anthrax," said Tshabalala.

South Africa's intelligence services were liaising with foreign countries in a bid to keep dangerous individuals away from South Africa during the summit, he added.

Tshabalala said the government expected 65,000 visitors to South Africa for the meeting - including around 40,000 activists from environmental, labour, youth and women's groups.

He said "thousands" of security agents would be deployed in Johannesburg for the event, but did not give an exact number.

Additional units would also be deployed to handle day-to-day crime in Johannesburg, considered one of the most dangerous cities in the world.

The summit aims to map out a set of action plans to reduce global poverty and the income gap between rich and poor nations without causing irreparable damage to the environment.

A concrete agenda for the summit has not yet been agreed after preparatory talks in Bali, Indonesia, collapsed.

African diplomats say that it is still not known whether U.S. President George W. Bush and British Prime Minister Tony Blair will be among visiting heads of state.