Dozens of people applauded as the soft-spoken Marcos, burdened with weapons and ammunition and puffing on a pipe through a mouth-slit in his black balaclava, mocked President Carlos Salinas de Gortari and called for a transitional government leading to clean elections. Troops and police guarding the palace door puffed on cigarettes, oblivious to the commotion.
Marcos was, after all, only an image on a television set propped on two chairs and a table, his voice crackling out from two old-fashioned loudhailers to a seated crowd of some 100 people. He was speaking on a video filmed in the dense Lacandon jungle, in the south-eastern state of Chiapas.
Had President Salinas appeared on the palace's central balcony, some 30 yards away, he might have passed unnoticed. The audience was riveted to the screen, alongside a large colour portrait of the revolutionary leader Emiliano Zapata, set up by civilian supporters of Marcos's Zapatista National Liberation Army (EZLN).
In many capitals, the scene would have been inconceivable. Here, Mr Salinas's ruling Institutional Revolutionary Party (PRI), has long been so sure of its grip on power as to allow such demonstrations of dissent. That may not hold true much longer.
The pro-EZLN civilians, who have also set up an exhibition of photos of Marcos and the January Chiapas rebellion, are calling for, and obtaining widespread support for, the guerrillas and their demand for an end to what is effectively a one-party system. 'The people are with the EZLN. They are not alone,' say pamphlets, which are eagerly snapped up.
Thousands of passers-by have filled in a questionnaire that asks whether they agree that Mr Salinas and his government should resign before presidential elections on 21 August. The majority have answered 'yes'.
The pro-EZLN group, led by three lawyers representing the guerrillas for free, intends to continue its provocative photo and video show until 10 April, a date that may prove to be the next big headache for a President stunned by the Chiapas rebellion, the kidnapping of the head of Mexico's biggest bank and the assassination of Mr Salinas's close friend, Luis Donaldo Colosio, the PRI presidential candidate.
It was on 10 April, 75 years ago, that Emiliano Zapata was lured into a trap and riddled with bullets by those who hijacked the revolution that had been largely based on his struggle for land reforms. Arguably, it was the date on which revolution began giving way to institution and the PRI's ensuing 65 years in power.
With many anti-government groups calling for marches to the zocalo on 10 April, Mr Salinas faces the prospect of the biggest anti-government demonstration in recent times. In the current atmosphere of instability and mistrust, fuelled by suspicions that Colosio's death may have been part of a PRI conspiracy, the zocalo that day could prove to be a powder keg.
Add the fact that Marcos has frozen negotiations with the government, accused the army of enforcing an economic blockade on parts of Chiapas and put his men on 'red alert'. A new EZLN strike would provide Mr Salinas with a grave dilemma. Despite the usual public display of unity, Mr Salinas's party is badly split and is showing signs of decomposing. The President has been humbled by a small band of mainly Indian peasant guerrillas and is poorly placed in his party's power struggle.
For the first time, after this week's nomination of an inexperienced technocrat, Ernesto Zedillo, 42, as substitute presidential candidate, some PRI officials concede privately that 65 years of uninterrupted rule could end with the presidential election.
Some are said to be quietly negotiating an 'amnesty' for Mr Salinas and his government in the event of defeat. His populist opponents blame him for concentrating on neo- liberal economic policies and the North American Free Trade Agreement (Nafta) while the vast majority of Mexicans were becoming poorer, hungrier and angrier.
The PRI's main opposition on 21 August will be Cuauhtemoc Cardenas, can didate of the Party of Democratic Revolution (PRD) and son of General Lazaro Cardenas, who, from 1934 to 1940, was probably Mexico's most popular president. But the languid Mr Cardenas may be his own worst enemy. He, and many Mexicans, believe he beat Mr Salinas in the 1988 presidential race and that the PRI-controlled Electoral Commission resorted to what is known here as 'electoral alchemy' to give Mr Salinas a narrow victory.
Mr Cardenas has never recognised Mr Salinas as president but, after several weeks of post-election tension, backed away from a confrontation. Non-recognition of Mr Salinas was one of the banners taken up by the EZLN.
Ending their six-year terms as multi-millionaires without any apparent effort has long been a tradition for Mexican presidents. Political analysts here estimate Mr Salinas will be worth hundreds of millions of dollars when, still in his forties, he hands over power at the end of the year. That is assuming everything passes peacefully between now and then.
In the meantime, it is no mere cliche that the poor are getting poorer. Economists estimate that 95 per cent of Mexico's wealth remains in the hands of a few hundred thousand people. Hence the Chiapas rebellion, which has already shown signs of spreading - notably in other poor states such as Guerrero and Oaxaca.
While land disputes are blamed for such unrest, Mexico's astonishingly persistent racism is a key underlying factor. Although there are 10 million indigenous people among the country's 80 million inhabitants, and the majority of the rest are of mixed race, you will have to look long and hard before you see a non-white face among the ruling class.
Mexico's indigenous people are not so much maltreated - though that, too, occurs - but, in a sense even worse, they are simply ignored.