Fares on most Metro buses and subways will rise by a nickel or more on Dec. 6 if a staff proposal submitted yesterday to the transit system's board is adopted, as appears likely. It would be Metro's third fare increase in 16 months.

Overall, the fare package would raise the cost of an average ride in the area by 12 percent, moving most base fares from 60 to 65 cents. Mileage fees that determine rush-hour rail fares would be raised, as would "crossing charges" for bus rides between jurisdictions.

The new fares would reduce ridership by about 2 percent, according to Metro planner Robert Pickett, accelerating a downward trend in patronage that has emerged in the last year. But revenues would still increase by the $7 million that Metro needs to balance its budget for the fiscal year 1982, Pickett said.

The Metro board plans to vote on the package on Oct. 29. Board members from Virginia and D.C. said they would support the plan's basic fare structure, although they opposed a 10-cent charge proposed for bus transfers.

The fare plan, however, conflicts markedly with what Maryland has sought. Its board members have argued for higher base fares and lower mileage fees for rail, which would hold down fares for commuter trips from the suburbs. Board members from Prince George's and Montgomery counties said yesterday they did not yet know how they would vote on the plan. For a fare increase to be approved, four of the six board members must vote for it, including at least one from each jurisdiction.

The board approved the pending fare increase in principle earlier this year as part of Metro's fiscal 1982 budget and ordered public hearings to determine how much it should be. Whatever rates are approved, they will take effect on Dec. 6, the day after the Red Line is opened through to the Van Ness station.

Inflation, dwindling ridership and Reagan administration plans to phase out federal operating subsidies have put Metro in an increasingly tight financial position, proponents of higher fares say.

"I'm not comfortable at this point with raising fares," board chairman Joseph Alexander said. "However, I don't see any alternative."

Metro officials said that the staff proposal offered somewhat lower fares than those used as the basis for the recently concluded public hearings.

Under the staff's plan, average rail fares would go up by 8 to 9 percent. The base fare in off-peak hours would be 65 cents, with 13.5 cents being added to that for each mile after three miles during rush hour. (The current rate is 12.5 cents.)

This would raise a rush-hour fare between Metro Center and Silver Spring to $1.25 from $1.20. The New Carrollton-Metro Center fare would rise to $1.65 from $1.50. Rides to Metro Center from National Airport and Addison Road would both rise by 10 cents.

Bus fares, meanwhile, would rise on the average something over 12 percent. Riders would pay a rush-hour base fare of 65 cents for trips that did not cross jurisdictional lines. The peak-hour District-Maryland Zone 1 charge would rise from $1.10 to $1.25, with the Zone 2 fare rising 10 cents to $1.50.

The District-Virginia Zone G crossing charge would go up from 60 cents to 65 cents, raising the cost of rush-hour trips on Metrobus to and from the inner communities of Northern Virginia from $1.20 to $1.30.

For off peak-hours, bus rides within Virginia and Maryland would cost a flat 65 cents while in the District the rides would continue to cost 60 cents. Off-peak rides between D.C. and Maryland would move from 85 cents to $1 and between D.C. and Virginia $1.20 to $1.30.

In addition, the package would charge riders 10 cents for a bus transfer. Metro officials argue this would curb a practice in which passengers give the currently free transfers to other people, who then board the buses for free. The District argues that this in effect would add 10 cents to the base fares of anyone who needed to transfer.

Alexander, the board chairman, predicted that "the bus transfer charge is not going to fly at this point" due to the District's opposition.

The plan would also raise the rail-to-bus transfer charge from 25 cents in the District to 40 cents, and from 35 cents in Virginia to 40 cents. D.C. would continue to have no transfer charge.

The plan would raise prices for some Flash passes. It would retain current practices of charging elderly and handicapped rail passengers one-half of peak fares and would raise their special fares on buses. It would retain free parking on Metro's lots on Saturdays.

In other Metro business before the board yesterday:

* D.C. Mayor Marion Barry formally proposed that elderly and handicapped people ride free on Metro at all times. Currently about 73,000 D.C. residents are riding at discounted fares available to those groups. Barry said the city government would cover the cost of eliminating the fares, an estimated $1.5 million a year.

* Metro General Manager Richard Page laid out a long-range capital program for the bus fleet that favors concentrating dwindling federal aid on rebuilding old buses in the fleet and halting the purchase of new ones. It also proposed consolidating bus maintenance into eight facilities scattered around the area.

* Operations staff requested more resources for cleaning and removing graffiti in buses and rail cars. Currently buses get a thorough cleaning inside and out every six months. The staff suggested doing it every two months.