American Telphone &Telegraph Co. for the first time yesterday appeared to take a stand against pending legislation that would alter its structure dramatically.

Although the Bell System's carefullly worded statement suggested the telephone giant had difficulties with bills before both the Senate and House Commerce committees, most knowledgable observers suggested provisions in the Senate bill that would permit the Federal Communications Commission to administer the restructuring of the Bell System were the crux of the AT&T objection.

At the same time, however, the Carter administration and the FCC, in separate letters from Commerce Secretary Philip Klutznick and FCC Chairman Charles Ferris, yesterday endorsed the thurst of the Senate bill that many observers say may be headed for passage by the Commerce Committee on Tuesday.

The flurry of activity came as supporters and critics of congressional efforts to revamp AT&T's structure prepared to face the next legislative test of what House and Senate advocates of the bills call communications deregulation.

But Pic Wagner, a spokesman for AT&T, which has been saying little publicly about the bills other than to praise Congress for taking up the issue, cautiously criticized both bills yesterday.

"We have considerable problems with the some of the language in both bills -- the one which emerged from the House communications subcommittee and also the one before the Senate," he said.

"In particular, we are concerned with provisions dealing with the telecommunications industry and the Bell System structure and those provisions pertaining to the authority of the FCC," he said, noting that the Bell System still is deeply in favor of a "workable bill." AT&T officials repeatedly have said it is critical that this Congress pass legislation that would permit the company to enter new, unregulated markets.

At the same time, however, Klutznick, in a letter to Sen. Howard Cannon (D-Nev.), chairman of the Senate Commerce Committee, yesterday urged "prompt action" on the telephone section of the Senate bill while at the same time recommending some legislative changes.

"The bill generally sets forth firm guidelines but wisely retains the commission's present necessary discretion to act on the basis of "experience," Klutznick wrote.

Chances of passage of the Senate bill -- which was introduced just nine days ago after months of internal Commerce Committee negotiations -- appear bright, particularly because the bill has the support of Cannon, Sen. Robert Packwood (R-Ore.), the panel's ranking minority member, Sen. Ernest Hollings (D-S.C.) and two other Republican members.

Klutznick suggested that the two-year period in the Senate bill for telephone terminal deregulation is too short and recommended that the committee set an overall period of eight years to complete the process.

In addition, Klutznick, who oversees the National Telecommunications and Information Administration, recommended that Senate bill identify only AT&T as a "dominant regulated" carrier and not all phone companies serving 750,000 or more people.

The Klutznick letter did not address the substantial changes in broadcasting regulation proposed in the Senate legislation, instead saying that administration policies on those matters still are being developed.

The House communications subcommittee Wednesday easily passed a telecommunications bill that would restructure AT&T into two companies over a 10-year period and alter the FC's regulation of many facets of the telecommunications industry. That bill will go to the House Commerce Committee next month.

The Senate bill essentially frees AT&T from the constraints of a 1956 consent decree with the government that barred the telephone giant from using its size to dominate fields such as data processing. But the measure also gives the FCC authority not contained in the House bill.

Ferris, who has been an outspoken advocate of steps to deregulate both the broadcast and telecommunications industry, expressed "enthusiastic and wholehearted support" for the Senate bill.

Meanwhile, lobbying groups such as the American Newspaper Publishers Association are raising questions about the speed with which the bill is being put to a Senate Commerce Committee vote.