By Tom Gibb BBC Brazil correspondent

The Brazilian Government has sent troops to reinforce the police surrounding a ranch occupied by more than 500 families of Brazil's Landless Movement (MST).

The property is owned by President Fernando Henrique Cardoso's two sons.

Agrarian Development Minister Raul Juggman has called the occupation an act of terrorism.

The protesters say they will resist any attempt to shift them by force from the property, about 200 kilometres (125 miles) from the capital Brasilia.

Cardoso's government says the opposition is behind the invasion

The leader of the occupation says the families have brought enough food with them to last for a month.

MST leaders say they want the farm given to landless families.

They say they occupied the farm after the authorities refused to discuss demands to be given other areas of land and to have electricity and water connected to existing MST settlements.

Election issue

The Landless Movement has for years been campaigning for major land reform in Brazil, where the distribution of wealth is one of the most unequal in the world.

MST activists frequently clash with the authorities

The government has been using divide and rule tactics, giving out land to other peasant organisations, and doing its best to discredit the confrontational tactics used by the MST.

At the same time, it is still relatively common for leaders of the Landless Movement to be murdered, cases which the police usually fail to solve.

The issue of land is likely to play a significant part in the presidential election campaign this year.

The government has already accused the main opposition Workers' Party of being behind the occupation of the president's family farm, something the party strongly denied.

On Saturday, Luiz Inacio Lula da Silva won the Workers' Party nomination for elections scheduled for October.

It is the fourth time he will be standing for the Workers' Party in presidential elections, after the polls in 1989, 1994 and 1998.