Thousands of demonstrators plan to commemorate the sixth anniversary of the U.S. Supreme Court decision legalizing abortion today with protest rallies, symobilic red roses, strident rhetoric, religious gatherings and a midday march along Pennsylvania Avenue.

For abortion opponents, today's central event will be a "March for Life' scheduled to begin at noon at the Ellipse and proceed to the White House and then along Pennsylvania Avenue to the Capitol, where it will end in an afternoon of rallying and congressional lobbying.

For supporters of the Supreme Court's abortion ruling, today will be marked by a series of news conferences and speeches intended to counter the antiabortion protests. Among proponents of legalized abortion will be leaders of the National Organization for Women, the National Abortion Bights Action League and the Religious Coalition for Abortion Rights.

These contrapuntal exhortations will be the latest reverberations of the Jan. 22, 1973, Supreme Court ruling that struck down most states' antiabortion laws and held that a woman has an absolute right during her first three months of pregnancy to deciwhether to bear her child or seek an abortion. Between the third and seventh months of pregnancy, the court ruled, the states may regulate medical aspects of abortion. After the 26th or 27th week of pregnancy, it said, the states may forbid all abortions except those essential to protect a mother's life or health.

The anniversary of the ruling has been commemorated annually in Washington with protest demonstrations by major antiabortion groups. Police estimates of the number of antiabortion demonstrators here last year ranged from 26,000 to 70,000 persons. Leaders of the "March for Life" have often given higher estimates.

Yesterday, "March for Life" organizers said they expect tens of thousands of protestors to converge on Washington for today's rallies. Their D.C. police permit authorizes as many as 100,000 demonstrators, officials said.

By yesterday evening, two conflicting abortion rallies were already under way here. The "March for Life" organizers held a dinner meeting of clergymen and other supporters in a local gymnasium in what one official termed a "prayerful" atmosphere. Later last night, about 100 backers of the 1973 ruling staged a silent vigil outside Supreme Court headquarters. It was sponsored by the Virginia Organization to Keep Abortion Legal.

One key aim of the "March for Life" rallies and its congressional lobbying campaign is to press for what the group's leaders describe as a "mandatory human life amendment" to the U.S. Constitution. Such an amendment would assert a "paramount right to life" for every "human being from the moment of fertilization." It would prohibit all abortions.

The "March for Life" demonstrators plan -- as they have in the past -- to distribute red roses to members of Congress and others as a "symbol of short life and martyrdom" and an emblem of their campaign for the "preborn child."

Organizers of groups supporting the Supreme Court abortion ruling say they intend to hold meetings today to consider ways of broadening options for legal abortions, including the possible establishment of funds to provide for abortions for indigent women.

The number of events planned for today here and across the nation by supporters of the Supreme Court's decision prompted some complaints last night from organizers of the "March for Life." "They realize that the prolife movement is growing. They're trying desperately to offset it. They've never had this kind of thing before," said Nellie J. Gray, "March for Life" president.

Davida Perry, public relations director for the Religious Coalition for Abortion Rights, disputed this assertion, saying that groups favoring the Supreme Court ruling had frequently staged Jan. 22 events in the past.