MANCHESTER, N.H. — Twenty residents of Manchester and surrounding towns have filed a lawsuit to stop the fluoridation of city water.

The case hinges on whether the city can distribute fluoridated water to six outlying communities, none of which voted to fluoridate their water.

The state requires approval by a referendum vote before fluoride can be put in consumers’ water. Manchester residents voted by a narrow margin in November 1999 to fluoridate water to prevent tooth decay.

All the legal requirements were followed when fluoridation was introduced, Mayor Robert Baines said. The program began about three months ago.

But consumers in Bedford, Goffstown, Hookset, Auburn, Londonderry and Derry also are connected to Manchester’s water system, although they did not vote for the fluoridation.

Not giving residents outside Manchester a chance to vote on fluoridation was illegal and unconstitutional, said Jed Z. Callen, a Concord attorney who represents the plaintiffs. Ten of the plaintiffs are from Manchester. The others are from Londonderry, Hookset, Auburn, Bedford and Goffstown.

Callen said the suit also challenges the state law that dictates the language for a fluoridation referendum. “It doesn’t allow Manchester to say, do you want arsenic and lead in your water? If people knew this they might have voted differently.”

Although Callen said the suit will concentrate on constitutional questions and not get bogged down in technical debates about the benefits or dangers of fluoridation, residents say the fluoride compound used by the Manchester Water Works contains contaminants including “measurable quantities of arsenic and lead.”

He said as soon as the city is served with the suit, he plans to ask the court for a preliminary injunction to turn off the fluoridation — something that can be done with a turn of a valve.