The Maryland attorney general's office said yesterday that the Montgomery County Council overstepped its bounds when it voted to prohibit local campaign contributions by employes of firms competing for the county cable franchise.

The council passed the restriction in late September in an effort to keep consideration of the cable franchise award free from the kinds of conflict-of-interest charges that have plagued other areas. After questions were raised about the legality of their action, council members requested a review by the attorney general's office.

In a letter released yesterday, Assistant Attorney General Mary N. Humphries said the prohibition amounts to a violation of first amendment rights of free speech and political activity.

Humphries also said that state election law, which would allow such contributions, takes precedence over any local ordinance.

Council members reached yesterday all said they would voluntarily refuse to accept contributions to their campaigns next year from persons they know to be working for cable firms competing for the franchise. Council member Neal Potter, calling the ruling "sensible," said that as long as campaign contributions must be made public, county politicians may be scared away from big-money donations from cable firm employes.

Edmond F. Rovner, a spokesman for County Executive Charles Gilchrist, said Gilchrist has already returned some contributions from cable firm employes, and will continue to do so even without the law. "We have lived by this and will continue to live by it," he said.

Even the cable firm representatives, many of them among the cadre of politically active local lawyers who bankroll county campaigns, said they would continue to abide by the spirit of the law.

"My company has instructed me not to contribute," said Stanton J. Gildenhorn, chairman of the county's Democratic Central Committee and the lawyer representing Tribune/Rogers Cablesystems, Inc. of Montgomery County, one of the many competing firms that has hired a prominent local attorney.

The ruling came on the very day that the county officially sent out its request for cable firms to begin fashioning proposals for the Montgomery franchise, considered one of the most coveted in the Washington area. By yesterday afternoon, 10 cable firms had picked up the lengthy and detailed proposal requests, officially entering the intense and expensive competition for the multimillion dollar contract.

Those firms now have three months to draw up their proposals.

The restriction on campaign contributions was attached to the official request forms and applied to every company wishing to enter the competition. Employes of the competing firms -- from lobbyists to the lowest-level clerks -- would be prohibited from making campaign contributions through 1983, and employes of the firm that wins the franchise would be restricted from making campaign contributions for the entire 15-year life of the contract.

County officials said they expect the franchise to be awarded by June.

The attorney general's ruling essentially upheld an earlier opinion by the county's lawyer and by Gilchrist, who had warned the council that they were overstepping their authority by restricting all employes of cable firms from making campaign contributions until 1983.

Gilchrist suggested that the council merely pass a resolution that he and all the council members would agree voluntarily not to take large campaign contributions from cable firm employes.

But the council members, anxious to avoid even the appearance of a conflict, made the contribution restriction a requirement for their approval of the county's cable proposal document.

Rovner said Gilchrist still backs the idea of a non-binding council resolution discouraging campaign donations from cable employes.

But some council members said privately that with so many politically active lawyers now working for various cable firms, a voluntary resolution against campaign contributions will be harder to enforce than a legal prohibition.

"You can't beat these people off with a stick," said one source close to the council.

Gilchrist has said that he will select whichever cable firm is recommended by the county's professional consultant and a citizens' panel, which will rank applicants based on an elaborate scoring system. In the event of a tie, however, Gilchrist would have to make the choice.