Just 18 hours before Chileans are to vote in a plebiscite on the rule of President Augusto Pinochet, his military government changed the rules so that voting no longer will be obligatory.

With long dormant political passions mounting, Gen. Pinochet appeared today to be seeking to divide the opposition and to reduce the number of "no" votes in Wednesday's polling.

Pinochet called for the vote Dec. 21 in response to U.N. resolution condemning Chile for human rights violations. The voters are being asked to affirm their support of Pinochet's government and to denounce "international aggression" against Chile.

Political parties, officially banned or suspended by the military since it seized power in 1973, were not prevented from circulating their call for followers to oppose Pinochet through abstention or - if forced to vote - with ballots marked "no."

Until today, however, the government said all residents over 18 had to vote. The national identity cards were to be marked and those unmarked would be invalidated. The cards are necessary for all dealings with the government in Chile.

The Christian Democratic Party that ruled from 1964 to 1970, and the Marxist Popular Unity coalition that followed it in power, first told followers to abstain. Faced with the tight rules against abstention, they changed and called for "no" votes.

With the plebiscite so hastily arranged and counting procedures in utter confusion, political observers were divided on how results could be interpreted - or if they could be trusted.

There was no discussion on such subtleties in the Santiago streets. For the 10th day, demonstrators shouting "Vote no" paraded downtown.Several persons were injured as police charged crowds near the Roman Catholic cathedral at noontime.