PORTLAND — As promised, the Friends of Congress Square Park sued Portland Wednesday in an effort to launch a citizens initiative to block the selling of part of the city-owned plaza to private developers.

On Sept. 16, the Portland City Council voted 6-3 to sell three-quarters of Congress Square Park to RockBridge Capital LLC and New Castle Hotels & Resorts, which are nearing completion of a $40 million renovation of the former Eastland Park Hotel. The developers have agreed to pay nearly $524,000 for a 9,500-square-foot section of the square, which they want to use to expand the hotel’s meeting space. The remaining 4,800-square-foot section would remain public space.

The week before the council vote, the Friends of Congress Square Park filed a petition to launch a citizens initiative that, if approved by voters, would alter the city code in hopes of making it more difficult to sell public spaces such as the Congress Square site.

The city’s lawyer rejected that petition on Sept. 13, denying the group’s right to launch such an initiative because it conflicts with state law and the city’s charter.

“Based on our attorneys’ advice, we are confident that a court will overturn this decision, as none of the city’s stated reasons for rejection withstand scrutiny,” the group said in a news release.

The group’s legal team consists of Robert H. Levin and Sarah M. McDaniel, according to the release.

The citizens initiative would specifically seek to strengthen the city’s Land Bank ordinance. The Land Bank is meant to protect natural and undeveloped properties in the city. It currently includes 25 such properties. The citizens initiative would create a new Land Bank category — “urban open public space” — and add 35 such properties, including Congress Square Park. Other well-known parks that would be added to the Land Bank include Fort Gorges, Monument Square, Longfellow Square and Tommy’s Park.

The initiative would also make it harder to sell a property in the Land Bank by requiring a supermajority City Council vote of at least eight out of nine votes or at least six City Council votes and voter approval in a municipal election.

“Our parks and open space are vitally important to our identity and quality of life,” said Frank Turek, president of the Friends of Congress Square Park. “It’s disappointing that the city is putting up roadblocks to a citizens’ initiative that will benefit all Portlanders.”

The group has asked the court to expedite the case so it can immediately proceed with the signature-gathering process.