A proposed consitution, the full text of which has not yet been made public, will be submitted to a national plebiscite on Sept. 11, the seventh anniversary of the military coup that ended democratic rule in the country, Gen. Augusto Pinochet announced tonight.

Pinochet, who serves as Chile's president, said the governing junta had added last minute changes to the draft constitution that will allow the armed forces, possibly under Pinochet's presidency, to rule for 16 more years and put off congressional elections until 1989.

Speaking on national televison, Pinochet said the new constitution, if approved, will go into effect six months after the election and that he would remain in office for an eight-year presidential term. After that, he said, the junta would reserve the right to nominate another president in 1989 to be approved by plebiscite and govern until 1997.

Pinochet said the full text of the consititution will be published Monday.

Opposition spokesmen immediately rejected the measure. "It is the consolidation of the dictatorship," said former Christian Democratic senator and party president Patricio Alwyn, one of the government's most outspoken critics. He called the "transition" period of 16 years a "farce and joke" while Chileans would be ruled by continuing the state of emergency and without political freedoms.

Pinochet said the authoritarian constitution is designed to guarantee that Marxist ideologies do not surface again in Chile.

Chile was governed from 1970 to 1973 by the leftist Popular Unity coalition led by President Salvador Allende, the first Marxist ever to reach such an office in free elections.

Pinochet's critics charge that the present lack of electoral guarantees and the predominance of government sponsored indicate that the announced vote on Sept. 11 will resemble a January 1978 plebiscite, also announced on short notice, which Pinochet won by a 3-to-1 margin.