The BBC's John MacLean in Manila: "A warning to President Estrada"

Led by former president Corazon Aquino and Cardinal Jaime Sin, Archbishop of Manila, they marched through the streets of the capital's financial district, Makati.

Police said about 25,000 people joined the demonstration, with thousands more watching from high-rise office blocks.

Mrs Aquino and Cardinal Sin told the crowd that proposed changes to the constitution do not need to be implemented.

Mr Estrada has asked Congress to amend the constitution to remove provisions which, he says, deter foreign investors, such as a ban on foreign ownership of land.

Cardinal Sin: "Don't trifle with democracy"

But critics allege other amendments are aimed at extending the maximum presidential term, which was limited to six years in 1987 after the removal of President Ferdinand Marcos.

Cardinal Sin said constitutional changes were not needed to foster economic growth, blaming economic problems on what he called the "corruption" of Mr Estrada's government.

"Constitutional constraints are not the cause of our economic problems," he told the crowd.

"It is caused by corruption in government. Businessmen's money is going to waste due to bribery."

He criticised Mr Estrada for going back on his inaugural pledge to protect the constitution, and said he would use the church's influence to block the plans.

"We will not allow anyone to trifle with our democracy," he said.

People power

Mrs Aquino and Cardinal Sin headed the People Power Revolution that overthrew the regime of President Marcos in 1986.

The rally's organisers believe that some of the abuses committed by the Marcos regime are in danger of reappearing under President Estrada, says the BBC's John McLean in Manila.

The Makati demonstration was a deliberate revival of the People Power phenomenon that deposed President Marcos.

Mrs Aquino is particularly concerned at the increasing influence of the family and associates of Mr Marcos.

She told the rally: "The issue is trust, not in the president but in the company he keeps."

Cronyism

Mrs Aquino and her followers believe that there has been a return to economic cronyism, that the freedom of the press is under attack and that the president's plans to amend the constitution have a nefarious purpose.