MEXICO CITY, NOV. 3 -- A coalition of opposition leaders of both right and left said today that they will jointly combat fraud and government favoritism toward ruling party candidates in next year's presidential election.

Speaking at a crowded press conference, 16 civic leaders and minority party activists announced the formation of a Democratic Assembly for Effective Sufferage that they said would monitor voting procedures and lobby for broader media coverage of opposition candidacies.

This is the first time that antigovernment politicians of the left and right have formally joined forces to "defend the vote" in a national election.

In several recent state contests coordinated protests from these ideologically hostile factions have lent greater credence to the fraud allegations that historically accompany Mexican elections.

"We have to defend everybody's votes, not just our own," said Arnulfo Martinez Verdugo, the former head of the Soviet-line Communist Party, which has merged with non-Marxist leftist groups to form the Mexican Socialist Party. "Democracy won't in itself solve all our problems, but it is an indispensable step towards addressing those problems."

Flanking Martinez on the dais were opposition activists ranging from right-wing businessmen to the congressional leader of a small Trotskyist faction to a one-time president of Mexico's ruling Institutional Revolutionary Party, the PRI. Although the assembly members said they were participating in the new organization as individuials, they include officials of all but one of Mexico's registered opposition parties.

The organization's name -- the demand for "effective sufferage" was a rallying cry of the 1910 Mexican Revolution -- represents a calculated challenge to the ruling party's claim to be the revolution's legitimate political heir.

"The need for effective sufferage is as urgent now as it was then," Porfirio Munoz Ledo, a former chairman of the ruling party, said today.

Norberto Corella, a ranking political organizer in the National Action Party, the largest and most conservative of Mexico's main oppposition groups, likened the new Democratic Assembly to the watchdog group in the Phillipines that denounced fraud in the last elections Ferdinand Marcos claimed to have won.

Carlos Salinas de Gortari, who is to be nominated formally Sunday as the PRI's presidential candidate and is virtually ensured of victory in the July 1988 election, has also pledged support for clean elections.