Thousands of protesting students marched through the heart of the capital tonight on the eve of a constitutional plebiscite that Chile's 7-year-old military government expects will extend its power for at least another nine years.

The antigovernment demonstrations were the largest seen here since troops led by Gen. Augusto Pinochet toppled democratically elected president Salvador Allende.

Outnumbered police wielding billy clubs chased the students through the city center, breaking up concentrations only to have them regroup to chant again several blocks away.

Thursday's plebiscite obliges every Chilean over the age of 18 to vote for or against a draft constitution written at President Pinochet's behest in an effort to legitimize his rule. Until the 1973 coup, Chile was South America's principal democracy.

Since Pinochet unexpectedly announced the plebiscite a month ago, Chile's newspapers and airwaves have been filled with a barrage of government propaganda suggesting that only the new constitution and Pinochet stand between the country and chaos.

Government-paid advertisements and commercials have focused on the economic troubles and near civil war that marked the final months of Allende's rule.

"In order never to return to disorder and street violence, yes to the constitution of liberty," said a full-page ad in El Mercurio, Santiago's most influential newspaper, last week.

"In order to return to the destruction of our sources of work , yes to the constitution of liberty," said a full-page ad, bought by the government, in El Mercurio today. Pinochet's opponents calculate that the government has spent $30,000 a day in El Mercurio alone since the plebiscite was announced.

Pinochet and government ministers have made speeches such as that by Alvara Bardon, the president of the Central Bank, who warned yesterday that the economy will face a "catastrophe" if the new constitution is not approved.

These warnings receive generous coverage by Chile's newspapers, most of which are progovernment, and television stations, all of which are government-controlled.

On Monday, Chile even provoked a diplomatic incident with neighboring Argentina, landing a helicopter on an oil exploration platform off a disputed part of Tierra del Fuego, apparently to rally Chile's strongly nationalist population around Pinochet and his regime.

According to Pinochet and Jaime Guzman, his chief political strategist, the purpose of the new constitution is to provide Chile with a "protected democracy" once the new document takes effect in 1989. Until then, Pinochet would remain in power during a transistion period to begin next year.

However, the regime has left itself an important escape hatch: in 1989, the ruling military junta may decide to name another president for eight more years and postpone promised elections. If that should turn out to be the case, the junt's decision would be submitted to another plebiscite.

Pinochet's opponents, led by the centrist Christian Democratic Party of ex-president Eduardo Frei, say that the real purpose of the vote is simply to institutionalize the dictatorship.

In the one major speech he was allowed to give, Frei called on the approximately 5.5 million voters to reject the constitution. He advocated instead a three-year transition under a military-civilian government.

Pinochet rejected that idea just as he rejected Frei's proposal that the two debate the merits of Pinochet's constitution on television. The president reportedly told friends that his agreeing to a debate with the urbane and intellectual Frei would be the same as Frei agreeing to a duel with the general, an expert marksman.

The Christian Democrats and other opposition parties officially are proscribed under the state of emergency that has been in force since the coup. The government announced today another six-month extension of the emergency. Opponents have been systematically denied the right to hold public meeting or argue their case against the constitution on television.

Genaro Arriagada a member of the Christian Democrats' political commission and cheif of the party's campaign against the constitution, declared:

"The people are under very strong pressure. First, they see only the government's propaganda and, second, they are being told that if 'no' wins, the country will return to the chaos" of the last months of the Allende government.

Arriagada said the opposition was not specifically prohibited from placing ads but the resources simply do not exist for more than one or two a week.

In any case, both the Roman Catholic Church and the Christian Democrats indicate fear that the plebiscite have been rigged, pointing out that the government has designed a ballot-counting system that provides no safeguards against massive fraud if that is necessary to ensure an outcome favorable to the government.

Andres Zaldivar, president of the Christian Democratic Party, said he believes Pinochet is determined to win the plebiscite at any cost "to become president for life. In effect, he's asking Chileans to put their lives and liberty in his hands alone."

Zaldivar said he estimates that anywhere from 20 to 30 percent of the voters favor Pinochet. But Zaldivar said he had been told the constitution will be approved by a vote of between 62 and 65 percent.