By Andrew Hurst

MEXICO CITY, Sept 2 (Reuters) -- It was one of those defining moments in Mexican politics, when the ground appeared to shift under one of the most venerated institutions of the state: the presidency.

President Ernesto Zedillo had barely sat down after delivering his last state-of-the-nation address before presidential elections next year when the president of Congress got up to give what was expected to be a sedate reply.

Instead of uttering a string of mild criticisms, Carlos Medina Plascencia, an opposition leader, delivered a withering attack on Zedillo's record in office.

The result has been uproar in Mexico's political Establishment.

In most Western democracies, Medina's remarks would have barely registered in the daily infighting of adversarial politics, but in Mexico, where strict protocol still surrounds the presidency, they had real shock value.

Although the power of the presidency has been greatly diminished since its heyday in the 1950s and 1960s, Mexicans still respect the institution.

Medina accused Zedillo of engaging in "totally vacuous rhetoric" while millions of Mexicans sank deeper into poverty.

"It's pointless to hear once more that everything is going fine if poverty is increasing dangerously in the country," he said. He also accused Zedillo of bad faith in his dealings with Congress, where the opposition holds a majority.

Legislators from Zedillo's long-ruling Institutional Revolutionary party (PRI) have accused Medina of showing gross disrespect for the office of the president and have demanded his resignation as head of Congress.

Medina is a member of the conservative National Action Party (PAN).

PRI congressmen heckled Medina mercilessly and forced him to interrupt his speech on several occasions.

And for some tense moments, it appeared that mayhem would break out on the floor of parliament as millions watched.

Guillermo Ortega, the main news anchorman on Mexico's biggest television network, Televisa, which traditionally leans toward the ruling PRI, had sharp words for Medina.

"What happened today in Congress was shameful," he told viewers. "One cannot understand a lack of respect such as what we saw on television," he said.

Some commentators say it was not so much what Medina said as the way he said it that was shocking in a country where extreme courtesy is the norm.

"He did violate some invisible standard of decorum in public debate while the PRI congressmen showed themselves to be intolerant," said Delal Baer, a specialist on Mexican affairs at Maryland's Johns Hopkins University.

Critics say Medina, as the representative of all parties in Congress, did not behave in a statesmanlike fashion because he exploited the situation for political gain.

"The opposition feels it has a real chance to win power next year, and it is not going to miss an opportunity to criticise the government," Baer said. "This is part of the destruction of the imperial presidency."

The PRI, which has ruled Mexico for 70 years, could face a real challenge next year if opposition parties form a coalition and field a single presidential candidate.

Others said Wednesday's incident may have been salutary.

"It's very important to consolidate the idea that the executive does not have a different level of credibility than the legislative branch," said Rod Camp, a political scientist at California's Claremont McKenna College. "Mexicans don't see them as equal institutions yet," he added.

Copyright 1999 Reuters Limited.