The citizens group that has been fighting Alexandria’s waterfront plan since last summer announced Thursday that it has collected enough signatures to force a supermajority vote by the City Council, but the city attorney said he doesn’t think the petition is valid.

Citizens for an Alternative ­Alexandria Waterfront Plan said that it has more than 200 signed petitions of protest from residential and commercial property owners who have land near the Potomac River. They hope to stop or delay the City Council from passing the waterfront redevelopment plan Saturday.

“The current city plan has been tainted by what we consider a cozy relationship with developers and a very poor relationship with citizens,” said Andrew ­McDonald, a co-founder of the citizens group. “The community is beginning to feel that this in­cred­ibly cozy relationship . . . has influenced their decision making.”

City Attorney James Banks said that Faroll Hamer, Alexandria’s director of planning and zoning, has the legal authority to decide whether the petition is valid. Banks said his advice would be to deny it.

“In my view, this is not even a close one,” he said. “It’s hard for me to believe that any experienced attorney could look at this” the way that the petitioners have.

The Alexandria waterfront on Feb. 23 , 2011. ( Photo by Jeffrey MacMillan/ For Washington Post ) (Jeffrey MacMillan/FOR THE WASHINGTON POST)

Hamer, who was not available for an interview, will probably issue her judgment just before the City Council starts its public hearing Saturday morning. More than 100 people have signed up to speak and more are expected.

McDonald on Thursday repeated previous charges, which city officials have strongly denied, that Mayor William D. Euille and Vice Mayor Kerry Donley have conflicts of interest. Euille is in business with someone who is an investor in Virtue Feed and Grain, a restaurant adjacent to the waterfront. ­Donley is employed by the bank that gave Virtue a loan, although Donley has said he was not involved in the transaction.

Banks said neither man, nor any council member, has a conflict of interest.

Council member Frank Fannon is a 17-year member of the Old Dominion Boat Club, which is fighting the city’s waterfront effort. But McDonald said he didn’t consider that a conflict of interest.

Alexandria’s efforts to redevelop its waterfront started more than three years ago with a series of public meetings. When the city’s proposal was unveiled last spring by Hamer’s planning department, it called for new parks, removed the boat club’s private parking lot at the water’s edge, allowed the building of hotels where warehouses now stand and encouraged commercial ­development as well as a miles-long pedestrian walkway.

Opposition soon arose, with some residents charging that the plan allowed too much dense development that would create traffic congestion and privatized public space. The citizens group forced a rewrite of the original city plan, but wasn’t satisfied with the second version, either. Months of protests ensued.

The biggest source of contention is what to do about four parcels of private land along the Potomac, two of which are warehouses owned by Robinson, a subsidiary of The Washington Post Co. The citizens group ­objected to city discussions with Robinson and Post officials about the fate of the terminals’ land after the company sued the city over a previous rezoning.

Thursday’s last-ditch effort to force a supermajority comes as at least four of the seven council members have indicated that they are likely to approve the revised city plan.