In his toughest arms policy statement since Ronald Reagan's election, Soviet President Leonid Brezhnev has:
* Threatened to counter the planned deployment of new American missiles in West Europe with an ''analogous'' challenge to United States territory. If recent private remarks by ranking Soviet officials are any indication, he was suggesting a counterforce of submarine-based missiles deployed near American shores.
* Announced what he billed as two unilateral arms-control moves, both seen by Western diplomats here as a bid to appeal to West European disarmament advocates and to shift European attention from the Polish crisis.
It is these arms-control moves that are likely to get the immediate headlines. They include (1) a freeze on deployment of further SS-20 missiles in the European part of the Soviet Union until the current Geneva talks with the Americans yield a compromise or until the US goes ahead with deploying the new missiles in Western Europe; and (2) an undertaking by Moscow, barring ''a new aggravation of the international situation, to reduce a certain number of its medium-range missiles on its own initiative.''
But the focus of attention for many foreign diplomats here was Mr. Brezhnev's unprecedentedly tough warning of the consequences should the Reagan administration follow through with deployment of new missiles ''capable of striking targets on the territory of the Soviet Union.''
Such a move, Mr. Brezhnev told the national congress of the official Soviet trade union organization March 16, would pose ''a real, additional threat to our country and its allies from the United States.''
Though Moscow has offered a variety of nonspecific vows to deal with this ''new threat,'' Mr. Brezhnev's latest remarks were far more explicit. For the first time he mentioned a challenge to US territory.
''This (US deployment in Western Europe) would compel us to take retaliatory steps that would place the other side, including the United States itself, its own territory, in an analogous position,'' he said, adding: ''This should not be forgotten.''
Meanwhile, in unveiling his new pair of arms-control overtures, the Soviet leader was clearly playing to Washington's West European allies. He preceded them with a charge that the US was ''evading serious discussion'' at the current Geneva talks on limiting European-based missile forces.
He said NATO plans to base new US missiles in Western Europe, originally formulated under the Carter administration, were part of a US bid to turn the region into the target of a Soviet retaliatory strike ''in case of (nuclear) conflict.''
NATO leaders say the deployment of new US missiles, due to start by late 1983 , is to counter Soviet installation of some 300 sophisticated SS-20 missiles, most of them targeted on Europe and each carrying three nuclear warheads. The Soviets argue that even without additional US missiles in Europe, essential ''parity'' already exists.
As for the specific arms control overtures, Western diplomats here charge that the SS-20 moratorium would merely freeze Soviet superiority, while the conditional and nonspecific pledge to reduce missiles did not specify whether this meant SS-20s or less advanced models that some Western analysts say are already outdated.
(During a speech to the Oklahoma legislature, Mr. Reagan rejected the Soviet announcement as meaningless. ''We must reduce the existing levels,'' he said.)
Yet some diplomats added that the proposals might well score some public relations points for the Soviets in Western Europe, particularly among disarmament groups, which have quieted markedly since imposition of martial law in Poland.
Diplomats argued that even if the reduction pledge covered some SS-20s, official statements on the current arms talks suggested the missiles would not be destroyed, but simply moved into the Asian part of the Soviet Union. Given the weapons' range, this would not necessarily prevent them from reaching Western Europe.
In his threat against US territory, the Soviet leader did not say precisely what he had in mind. Some foreign analysts postulated that he had in mind undoing the Kennedy-Khrushchev understanding on placing nuclear missiles in Cuba. But few diplomats went as far as to suggest Mr. Brezhnev intended publicly to preview a new Cuban missile crisis.
Yet to the extent the Soviet Union is genuinely planning such a retaliatory move at this stage, ranking Soviet officials have suggested it would most likely take the form of a submarine-based counterforce. The officials have mentioned this as a possible response in private remarks to some diplomats--and to at least one group of visiting West European political figures.
Both the Brezhnev warning, and his latest arms-control overtures, were seen as reflecting a genuine drive for some sort of breakthrough in an East-West diplomatic relationship clouded by the Polish crisis.
The Soviet leader himself, as he did in addressing the Communist Party congress here one year ago, suggested that economic troubles at home were one spur for this drive.
He offered his bleakest assessment to date of the domestic food situation and said ''urgent'' action was needed to combat shortages. He said arms expenditures were ''diverting considerable resources'' from economic projects. His display of public concern over what have become chronic shortages of some food commodities was unprecendented.
He termed the ''food situation'' a ''vital and urgent'' problem and then turned, with what some foreign analysts saw as an oddly apologetic note, to a ''food program'' first mentioned 17 months ago and expected to be announced formally this spring.