The United States and the Soviet Union formally opened talks here today on reducing their long-range nuclear missile and bomber forces, negotiations that President Reagan has called "one of the most important tasks of our age."

Reagan's assessment came in a letter to Edward Rowny, the chief U.S. delegate to these strategic arms reduction talks. Rowny read portions of the letter to his Soviet counterpart, Ambassador Viktor P. Karpov, during their first hour-long meeting today at the Soviet mission here.

Reagan said that despite more than a decade of arms talks, "nuclear weapons continue to accumulate and the strategic relationship between the Soviet Union and the United States has steadily become less stable"--a development he attributed to the "massive buildup" of Moscow's missile forces over the past 15 years.

Reagan said the two superpowers "are trustees for humanity in the great task of ending the menace of nuclear arsenals" and he said he was convinced this could be accomplished if both nations agree that "the only legitimate function of nuclear arms is to deter aggression."

Rowny, at a brief press conerence after the meeting, declined to describe Karpov's reaction to the letter, but the Soviet ambassador had also made a generally positive statement about the ultimate need for a new arms agreement when he arrived here Sunday.

Nevertheless, both sides enter these negotiations very far apart on how to reduce their arsenals and how to achieve what the U.S. side calls "equality" in atomic striking power and the Russians call "equal security."

Rowny and Karpov have known each other for more than six years, most of it as members of opposing delegations in the previous strategic arms limitation talks. Rowny said of today's largely procedural meeting that "it was typical Karpov-Rowny. All business and a little pleasantries."

While there are big obstacles to overcome, there are also forces pushing both sides toward an agreement. One is the huge cost of the weapons. Others involve economic problems in the Soviet Union and a feeling among some specialists that Moscow would like a breather in the competition with the United States.

There is also incentive for Moscow, as American officials see it, in their wanting to limit the deployment of the new U.S. weapons that could threaten their land-based missile forces. There is also a growing and politically unpredictable antinuclear movement in the West that is bringing pressure on the Reagan administration from both at home and Western Europe.

Today, Rowny told reporters that, after a recent tour of European capitals, he was told that the antinuclear movements had "peaked." He said it was his hope that "when people understand we are moving toward reducing the risks" of nuclear war that the emotion of the movement will be replaced because there will be "no disagreement between their goals and ours."

In his letter to Rowny, Reagan stressed what is another main point of contention with Moscow about how these talks should proceed. He said reductions "must immediately focus on the most destabilizing elements of the strategic balance," meaning the land-based missiles, which are both the most accurate and the most vulnerable to attack and thus, in the U.S. view, most destabilizing in a crisis.

The meeting today was limited to the two delegation chiefs. The first session of the full delegations will take place tomorrow and then the teams will meet every Tuesday and Thursday beginning next week.