MOSCOW -- Profound political changes are under way in the Soviet Union as the country prepares this month for its first contested parliamentary elections since the earliest days of the Soviet state.

A dramatic upsurge in political activism at the grass-roots level has followed President Mikhail S. Gorbachev's pledge that the Communist Party will end its 70-year monopoly on power and seek instead to play a leading--but no longer all-controlling--role in the government.

New constitutional structures--a powerful national parliament, an executive presidency, strengthened regional and local governments--should be in place within months of the March 26 elections, and Gorbachev has declared his determination to use them to push through further reforms.

A new political culture is being created at the same time, one that fosters popular participation, encourages debate, permits dissent and is already challenging the party.

Gorbachev calls the process "a vast school of democracy" for the country and sees it as the key to his overall effort to transform the entire Soviet political and economic system.

"It is already clear that we are not dealing with merely a formal procedure but with genuine elections of people's representatives to the highest organs of power," Gorbachev said as the country entered a month of formal campaigning. "This is an enormous step forward from the practice of previous years."

The changes are not aimed at turning the Soviet Union into a liberal Western democracy but at reshaping and redefining socialism. The Communist Party has made it clear that its interest is in sharing power, not forsaking it.

The election process has not been smooth. These are the Soviet Union's first multi-candidate, nationwide elections in memory. Most of the procedures have never been used before, and confusion has been widespread.

Those who fear the loss of power frequently have been ruthless in dealing with their opponents to ensure that they win--effectively demonstrating that real power is at stake.

Although the difficulties have discouraged many liberals who had hoped the elections would be "the dawn of democracy," as one writer put it, most of the country's senior political analysts rank the changes, in terms of potential, second only to the 1917 Revolution, which overthrew the czarist regime and brought the Bolsheviks to power.

Huge Impact on Country

"We are witnessing very great shifts in political consciousness, which the elections have awakened and brought to a high level," Prof. Boris Grushin, deputy director of the Center for Public Opinion Research, said last week in an interview. "There has been a huge impact across the country. Many things that once were so certain no longer are. The scale of change is hard to predict, and it is not irreversible.

"There have been procedural difficulties, to be sure, but we are also accumulating important experience, which I believe will expand and accelerate the process of change."

Extraordinary scenes, all the more dramatic after years of political passivity, have unfolded across the nation in the last two months of preliminary campaigning for the new Congress of People's Deputies.

Veteran party officials, whose word has virtually been law, have been cast aside in dozens of constituencies in favor of determined reformers--also party members but from the rank and file--as they stand for election to the congress.

Meetings that even three years ago would have been broken up by police as illegal "anti-Soviet agitation" now fill community centers, labor union halls and school auditoriums almost every night, drawing overflow crowds and continuing for 12 hours or more.

Party Policies Under Attack

Party policies, unchallengeable for so long as "the will of the working class," are under unprecedented attack--from workers, farmers and intellectuals alike and from within the party as much as from outside.

Even V. I. Lenin, the Bolshevik revolutionary who founded the Soviet state and who has been revered here as politically infallible, is being criticized. Lenin, it is said, pushed the party into its long monopoly on power, now regarded as the root of the country's problems.