Opposition plans to create a "grand coalition government" next year face a strong challenge from the left and from pro-government forces claiming a hidden victory in President Augusto Pinochet's defeat.

"It's going to be very, very hard," Arturo Valenzuela, director of Latin American studies at Georgetown University in Washington, said Saturday. The coalition must realize that next year's open election "is not in the bag," said Valenzuela, a Chilean who came home to monitor last week's vote on Pinochet.He said the coalition of 16 opposition parties from the center-right to Socialist left that campaigned for Pinochet's defeat must avoid overconfidence.

Pinochet lost by more than 800,000 votes in Wednesday's yes-or-no referendum on whether to extend his 15-year rule to 1997.

According to the 1980 constitution, he remains president for 16 more months. The winner of elections planned for December 1989 is to assume power in March 1990.

The opposition coalition's secretary-general, Genaro Arrigada, said he is not worried that the opposition will fall apart, despite the country's tradition for splitting into what Chileans call "the three thirds:" right, center and left. Most previous presidential elections had three major candidates, one from each camp.

"If we are going to have three thirds, it's not important if there is respect and understanding," he said.

Pinochet, 72, who seized power in a 1973 coup, has vowed to remain in office until his term runs out. The powerful army he commands has shown no willingness to bargain with politicians.