PORTLAND – Passengers on the Greater Portland Transit District’s Metro system rode for free Monday to celebrate the official arrival of clean-burning buses powered by compressed natural gas.

Fares were scrapped for the day on all routes as the new buses and new natural gas refueling station were introduced at a kickoff celebration hosted by Metro and organizations that promote the use of alternative fuels.

Thirteen of the system’s 28 buses now are powered with natural gas, while the older models run on diesel oil. Metro hopes eventually to have its entire fleet powered with the cleaner fuel, said Peter Cavanaugh, operations manager.

The new buses are the first in Maine to run on natural gas, although the seasonal Island Explorer service in Acadia National Park has propane-powered buses. Bangor also has reduced emissions by running all of its city buses on biodiesel fuel that is made of 20 percent vegetable oil and 80 percent petroleum.

Portland isn’t alone in turning to buses that run on natural gas. Steve Linnell of Maine Clean Cities said 25 percent of the nation’s buses coming off the assembly lines today run on the cleaner fuel.

“At this point, it’s a well-tested and proven technology,” he said. “They’re used in New York City. You go to California, they’re all over the place.”

Metro riders will notice a decrease in noise from the new buses, Cavanaugh said. As for economics, natural gas now has a slight cost advantage over diesel fuel, he said, while differences in maintenance costs between the old and new buses have yet to be determined.

The new white and light blue buses have been phased in over the past several weeks.

They’re easily spotted by the fuel tank on the top, which carries the message, “Metro runs on clean natural gas.”

The bus service will share the natural gas fueling station with the Portland School Department and will make the station available to fuel any other vehicles that use natural gas.

“We on the Metro board believe this is a major step in preparing our transportation system for the next generation of travelers,” said Jeffrey Monroe, chairman of the Metro board of directors.

Gov. John Baldacci, U.S. Rep. Tom Allen and the mayors of Portland and Westbrook were among the speakers at the kickoff event at Metro offices.

“We are excited to see clean fuel buses coming to Greater Portland Transit District,” Baldacci said.