By Tom Gibb BBC Correspondent in Brazil

Brazil's president, former union leader Luiz Inacio Lula da Silva, is facing his first major strike this week.

Civil servants lined the streets in Brasilia

Public sector unions have walked out, initially for seventy-two hours, in opposition to major changes in the country's pension system put forward by the government.

This is the start of a long campaign in which public sector unions are threatening to shut down much of the government unless reforms to the pension system are withdrawn from Congress.

Some universities and hospitals have been affected. Social security offices and other government departments have remained closed.

The reforms would place limits on the size of public sector pensions and raise the retirement age. They would also allow pensions to be taxed.

Lula's gamble

Economists say these changes are essential to prevent Brazil from going bankrupt.

Posters promoted the strike

They would also end long inequalities with private sector workers who receive lower pensions, despite paying similar contributions.

Lula, as the new president is known, has staked the success of his government on getting the reforms through.

But they are bitterly opposed by the left of his own Workers Party, as well as by many of his former colleagues in the unions.

Initial estimates suggest between 40-50% of public sector workers have gone on strike, including university professors and social security workers.

Gathering momentum

But news reports suggested the initial protests had failed to shut key federal offices.

Unions had vowed to paralyse Brazil's Central Bank adn Finance Ministry but in the capital Brasilia, most federal employees went to work.

Protestors remained adamant that the protests, expected to last at least three days, would gather momentum: