Hundreds of people have fired off complaints to Metro for running an ad campaign on its buses that questions the belief in God.

The transit system said the controversial ads have solicited just one compliment, while receiving 251 complaints.

“Why believe in a god?” say the American Humanist Association ads that went up on a handful of buses two weeks ago and inside the buses on Monday. “Just be good for goodness’ sake.”

The ads have sparked more ire than usual for Metro, said agency spokeswoman Candace Smith, even though advertising on buses and in stations has long been a legal morass for transit systems nationwide.

“As a public agency, Metro must observe the First Amendment with respect to the acceptance of commercial advertising,” Smith said. “Although we understand that feelings and perceptions will vary among individuals within the community, we cannot reject advertising because an individual, or group, finds it inappropriate or offensive.”

The transit system has lost lawsuits when it refused to run ads. It had to run posters suggesting that President Ronald Reagan led a “jelly bean republic” after losing a court decision in 1984.

Since then, Metro has allowed many campaigns that spark complaints, including recent ads for the “Fallout 3” video game that some deemed too violent. The transit agency’s ads cannot be factually misleading or false, nor can they violate laws or incite violence, Smith said. Profanity is also out. Everything else must be accepted.

For some, though, even the questioning of the existence of a god translates to obscenity. One person wrote to Metro, “That ad is obscene to me!? I wouldn’t want my children reading that.”

Another wrote of plans to complain to the American Civil Liberties Union on grounds that the ads violate a separation of church and state by a publicly funded organization.

The local branch of the ACLU said it had not received complaints about the campaign, nor would the free-speech advocates likely take on the case as they have successfully sued to keep such advertising open.

“The principle is as old as the hills,” said Fritz Mulhauser, staff attorney for the ACLU of the National Capital Area. “If Metro opens its space and walls to advertising, it cannot pick and choose.”

It’s not clear how many of those who complained actually ride the Metro system, as all but five complaints arrived via e-mail. One signed an e-mail as a “D.C. resident, Metro rider, and ‘BELIEVER’ in God,” while another writer acknowledged, “I have never had the privilege to actually visit Washington, D.C.”

Some of the letter writers said they learned of the campaign from or AOL and wrote in before the ads appeared on any buses.

The American Humanist Association began running 20 ads on the backs and sides of Metro buses starting Nov. 17 in Northwest and Southeast. Another 200 ads, which run inside behind the drivers’ seats, began Monday.

On Sunday, the Beltway Atheists plan to piggyback on the campaign and give out sweaters to homeless people in Dupont Circle to kick off of a week-long “be good for goodness’ sake” program.

The controversy has been a boon for the D.C. nonprofit American Humanist Association, which spent less than $12,000 on the bus ads. Since starting the campaign, 638 new members have signed up, spokesman Fred Edwords said. Traffic to its Web site spiked, Edwords said, and donors contributed thousands of dollars.

Metro spokeswoman Smith said the number of complaints represents a small fraction of its ridership, which averages more than a million trips on buses and trains daily.

“Do we think we’re losing customers over this?” Smith said. “I doubt it.”

She said Metro responds to each complaint, urging those who complain to contact the advertiser directly. Or, she said, “They can pony up money for counter advertising.”

She said no groups have contacted the agency to buy ads in response to the campaign, which ends in late December.