LOS ANGELES — Pot leaves are easy to find on Facebook pages. But the nation’s largest social-networking site has decided they cannot appear in advertisements, prohibiting them as “”illegal content.””

The policy was disclosed Tuesday after a national campaign promoting legalization accused Facebook of censoring political speech. The Just Say Now campaign said the popular website banned its ads after they had run for more than a week. The ads featured the readily recognizable leaf and asked the website’s users to “”sign the petition to President Obama to support states’ rights to legalize marijuana.””

“”We’re not allowed to show the image of the candidate that we are advancing,”” said Michael Whitney, with Firedoglake.com, a progressive blog that is part of the campaign. “”That’s why we’re calling out Facebook for this really backwards decision.””

Facebook stressed that it has not banned the ad promoting legalization, just the leaf. “”We’d like to reiterate that ‘Just Say Now’ can promote their campaign and petition through Facebook Ads as long as they use another image,”” said Annie Ta, a spokeswoman.

But Whitney said the ad with the pot leaf was twice as effective as one with Obama’s image. “”It was really critical to the success of our campaign,”” he said.

The campaign for Proposition 19, the initiative to legalize marijuana in California, has a Facebook page but the pot leaf is nowhere to be found. Instead, the campaign’s image is an orange-and-yellow sun, reminiscent of vintage packing crate labels for the state’s legal agricultural products.

“”It just symbolizes hope, change, new day, new horizon,”” said Dale Sky Jones, a campaign spokeswoman.

Jones said the campaign deliberately chose not to use a cannabis leaf or the color green because it could distract from the message that marijuana should be regulated and taxed, and might prevent the campaign from advertising with some media. “”It’s not that the leaf itself is an evil image. It’s that some folks have a negative connotation attached to it,”” she said.

The Marijuana Policy Project, which promotes legalization nationwide, also does not use the pot leaf. “”There is still so much stigma associated with that,”” said Mike Meno, a spokesman.

One of Facebook’s co-founders, Dustin Moskovitz, is a top donor to Proposition 19. He has contributed at least $20,000, according to campaign finance reports. Moskovitz, who now runs a software company, declined to comment.

Whitney said the Just Say Now campaign, a joint project of Firedoglake.com and Students for Sensible Drug Policy, ran the ads on Google for less than a day without concern from the company.

Diana Adair, a spokeswoman for the Mountain View, Calif., company, said she could not comment on specific advertisers. But she said, “”An ad for a political issue with an image depicting a marijuana leaf would likely be acceptable under our policies

Facebook, which is based in Palo Alto, Calif., offered shifting explanations for its decision. The Just Say Now campaign said the website informed the campaign last week that the image was not acceptable under its policy on “”smoking products.”” But no such policy is included in its advertising guidelines and Facebook did not respond to several requests for the policy.

On Tuesday, Ta said the pot leaf was excluded because Facebook does not allow images of drugs or drug paraphernalia in ads and then later said, “”Our advertising policies prohibit the paid promotion of illegal content.”” The policy says nothing specific about drugs or drug paraphernalia, but it does indicate that ads cannot contain or promote “”unlawful content”” or “”illegal activity.””

Ta did not respond to a request to explain how the image of a pot leaf, which is seemingly ubiquitious in popular culture, was illegal content.

“”We want Facebook to be a place where people can openly discuss issues and express their views, while respecting the rights and feelings of others,”” she wrote in an e-mail. “”Our team has worked with ‘Just Say Now’ directly and explained our policies in depth.””

Whitney said Facebook conveyed neither of Ta’s explanations. He said that before Facebook’s “”abrupt turnaround,”” a representative worked with the campaign on the ad and indicated the image was the most important element. “”Then they banned our logo,”” he said.

The ad was targeted to Facebook users whose demographics and interests suggested they might support legalization. Whitney said it was seen 38 million times, but he would not disclose how many Facebook users clicked it or how many signed the petition. “”We were bringing new people in the door and identifying people we could not otherwise have hoped to identify,”” he said.

Facebook has numerous pages that include images of marijuana leaves and buds, including some that are devoted to growing marijuana, which remains a federal crime. Ta noted that the website has different policies for its free users. One of the many responsibilities listed in that policy is that users “”will not use Facebook to do anything unlawful.””

The Just Say Now campaign also has a Facebook site and it uses the pot leaf logo. But the campaign has now partially covered it with a black bar reading: “”Censored.””