Montgomery County will increase by 40 percent the rates for long-term public parking in Bethesda and Silver Spring to help finance new parking projects for the two rapidly urbanizing jurisdictions, transportation officials said yesterday.

Montgomery Transportation Director Robert S. McGarry said that, starting Oct. 1, commuters who use the public spots will pay 35 cents an hour, 10 cents more than the current hourly rate. An increase of 5 cents an hour had been set to go into effect July 1, but county officials deferred it so that there would be just one adjustment to the parking meters.

The increase will mean it will cost $3.15 for nine hours, up from the current $2.25, and $4.20 for 12 hours, up from the current $3. Private parking rates in Silver Spring and Bethesda range from $4 to $5 a day. Short-term parking rates, 50 cents an hour for six hours or less, are not affected.

The increases are part of a series of changes implemented in March 1986 that were the first raises in eight years. The new parking rates caused an outcry, prompting more than 1,000 letters to county officials, who decided to phase in the increases.

McGarry said the county's financial consultants urged the higher rate to back the sale of $27 million in revenue bonds for parking space construction. He said the increases, which will affect about 6,800 spaces in Silver Spring and about 3,200 spaces in Bethesda, are not related to the county's efforts to encourage commuters to form car pools or to use mass transit rather than drive in the congested downtowns.

Parking revenue, fines and assessments on participating businesses are used for the planning, construction and maintenance of parking spaces in the county's special parking districts in Silver Spring, Bethesda, Wheaton and other locations. County Executive Sidney Kramer also is recommending that Silver Spring and Bethesda be designated as transportation management districts, where incentives would be offered for forming car pools and riding trains and buses.

Chamber of Commerce officials in Silver Spring and Bethesda said yesterday that increased parking rates are never welcome but that more parking is desperately needed.

Bethesda Chamber of Commerce President Robert Kelly said the new rates will affect people traveling to and working in Bethesda. "We are concerned," he said, "but we also recognize the problem {the county} has." Chamber studies indicate more parking is needed for Bethesda, he said.

Joe Kumper, president of the Silver Spring Chamber of Commerce, said he will meet with chamber officials to determine whether any action should be taken. But said he thought more parking is needed.

The $27 million revenue bond sale in June will be used for projects in Bethesda and Silver Spring, said Thomas Huff, chief of policy planning for the parking division. Included are two parking garages on Cameron Street in Silver Spring, and facilities in Bethesda near Old Georgetown Road and Woodmont Avenue, and on Elm Street.