Emmet Andrews was elected interim president of the 285,000-member American Postal Workers Union yesterday to fill the unexpired term of Francis S. Filbey, who died last Tuesday.

The death of Filbey and Andrews' election by the union's national executive board came at a time when the union is preparing to fight a proposal to cut mail delivery service to five days a week and amidst some internal division among union members.

Last week, several dozen union members from northeastern states picketed union headquarters here to protest what they claim is a lack of action by the national union in processing their grievances. They said they would start a recall campaign of all members of the executive board if their demands are not met.

Union leaders say that, despite the hiring of additional staff, they are overwhelmed with the 16,000 grievances they have received since the current contract was signed in 1975.

"It's an insurmountable task in a way," Andrews said yesterday. "We get about 250 (grievances) a week in here, and all those are supposed to be evaluated."

Grievances and "excessing" - the practice of moving postal employees from one location to another - are expected to be among the major issues when negotiations for a new contract with the U.S. Postal Service begin next spring. The current contract expires in July, 1978.

Andrew said after his election yesterday that his union - which represents clerical workers, maintenance personnel, drivers and special delivery employees, but not letter carriers - will mount an "all-out legislative program" to prevent the Postal Service from cutting deliveries to five days.