Defense Secretary Harold Brown signaled yesterday that the Carter administration has decided to swim against the tide of "the Russians are coming" warnings and make cuts in the $123.1 billion Pentagon budget proposed by the Ford administration.

Brown told the Senate Armed Services Committee that he plans to submit an amended Pentagon budget, with cuts in manpower and hardware programs, to Congress by Feb. 15 or shortly thereafter.

"I am looking at possible budget changes," he told the committee, although he refused to confirm or deny reports that the proposed cuts amount to $2.8 billion.

"There is a fairly extensive dialogue between myself and staff on each" of the proposed cuts, but "no decisions have been made," Brown said.

The new Defense Secretary, said for example, a non nuclear Lance missile has serious shortcomings, such as not being accurate enough to destroy the target with a TNT-type warhead; that perhaps the Marine Corps has gone too far in buying its own fighter planes rather than relying on the Navy's and that producing fewer B-1 bombers than the 244 the Air Force wants would not raise the price per plane an unacceptable amount.

Congressional and Pentagon sources said yesterday that the military services were directed last Friday to send their reports on the proposed cuts to Brown's office this week so he can submit his own recommendations to President Carter by Jan. 31.

The initial protests of several senators against proposed cuts signaled yesterday that the Carter administration is going to run head on into the argument that the United States dare not reduce its military budget in the face of the Soviet buildup.

Gen. George S. Brown, chairman of the Joint Chiefs of Staff, sat on Secretarys Brown's right through the committee's morning and afternoon sessions and was generally supportive of the secretary in his testimony.

Both Gen. Brown and Secretary Brown told the committee, for example, that Soviet civil defense activities do not threaten to upset the strategic balance between the United States and the Soviet Union by making nuclear war look tolerable to the better protected Russians.

"I think it has been overstated." said Gen. Brown in referring to recent published reports about the significance of the Soviet civil defense against a nuclear attack. "We are some years away from reaching the point where it is an unbalancing factor in a strategic balance equation," he said.

Secretary Brown, a nuclear weapons expert from his work as a nuclear physicist, said the U.S. offensive weapons could overwhelm a Soviet defense for its civilian population by the way the attack was "configured and launched."

Asked after the hearing whether the Soviet civil defense effort might be designed against a Chinese rather than an American Nuclear attack, Gen. Brown answered "I don't know." Secretary Brown replied: "I can't judge. It's certainly possible."

Both Browns stressed the importance of strengthening U.S. NATO nonnuclear forces so that there would be less likelihood of turning to nuclear missiles and bombs if war broke out. They also agreed on the necessity to maintain at the same time the balance between the United States and Russia in nuclear strategic weapons - land-based ICBMs, submarine missiles, and long-range bombers.

While agreeing on overall strategy, yesterday's hearing did display some difference between Defense Secretary and military leaders about the best weapons for carrying it out.

Killing the nonnuclear Lance Missile program to save $82 million is one of the proposed cuts on the list making the rounds of the Pentagon and some congressional offices. Sen. Howard W. Cannon (D-Nev.) protested yesterday that he supported a nonnuclear Lance only after the military told him it was very, very worthwhile" for NATO. Now, he said, he has learned it is on the list of possible cuts.

Secretary Brown, displaying an expertise in individual weapons that recent defense secretaries did not have, ticked off the shortcomings of Lance, including the question of whether was a conventional, rather than nuclear, warhead it could hit close enough to the target to destroy it. Gen. Brown, however, endorsed the change to a nonnuclear warhead.

Another proposal is to slice $300 million from the B-1 bomber production by turning out five instead of the planned eight planes in fiscal 1978. Brown insisted he had not made up his mind about the B-1 but said producing fewer planes would not push up the cost of each plane inordinately.

Congressional sources said other proposed cuts include eliminating three F-4 Marine fighter squadrons; stretching out the Air Force F-15 fighter production and M-X ICBM development: killing the Navy's strike cruiser; reducing Army manpower by 20,000 people and slicing $100 million from the Army's attack helicopter program.