Six and a half years after negotiations began, the Senate approved the landmark medium-range missile treaty with the Soviet Union Friday, marking history's first cutback in offensive nuclear weapons.

After systematically rejecting a last flurry of amendments, a whopping bipartisan majority voted 93-5 to approve the first nuclear arms accord between the superpowers since the 1972 Anti-Ballistic Missile Treaty signed by Richard Nixon and Leonid Brezhnev.The vote came just in time for the ratification documents to be completed at the White House and rushed to Moscow where President Reagan and Soviet leader Mikhail S. Gorbachev will conclude the agreement at the summit next week.

As the final tally was announced, crowds filling the visitors galleries above the Senate floor stood in prolonged applause.

Moments later, Majority Leader Robert C. Byrd, D-W.Va., and Minority Leader Bob Dole, R-Kan., telephoned news of the vote to Reagan in Helsinki, Finland, and were promptly invited to join the presidential party in Moscow.

In a statement released in Helsinki, Reagan said the overwhelming vote "clearly shows support for our arms-control objectives."

White House chief of staff Howard H. Baker Jr. is expected to depart for Moscow Saturday with the ratification documents. He will be on hand when Reagan arrives in the Soviet capital Sunday. Plans call for the two Senate leaders to go to Moscow early next week.

"This is really America's treaty," Dole declared after talking with the president. "It's in the best interest of the American people."

Byrd called the treaty's approval a testament to "the steadfastness of NATO" and to bipartisan cooperation in U.S. foreign policy.

Sen. Alan Cranston, D-Calif., said that while it eliminates only 4 percent of the superpowers' nuclear warheads, it "lays the foundation (to) enable us to move on . . . to other treaties that can substantially reduce the scale, cost and dangers of this arms race."

The margin of approval surprised even optimistic supporters, who had expected perhaps 10 senators to oppose the final resolution of ratification.

In the end, Sen. Jesse Helms, R-N.C., who led efforts to tack killer amendments to the treaty text, was joined only by Republican Sens. Gordon Humphrey of New Hampshire, Steve Symms of Idaho, and Malcolm Wallop of Wyoming, and a lone conservative Democrat, Sen. Ernest F. Hollings of South Carolina.

Despite the resounding endorsement, some Republicans remained angry over an amendment that asserted the Senate's right to interpret treaties, as well as over a late-night clash with Byrd Thursday. The Senate majority leader accused some Republican senators of bogging down ratification with "Mickey Mouse amendments" and implied he would block a vote if they persisted with proposals that threatened the interpretation provision he sponsored.

In a dispute over the 16-year-old anti-ballistic treaty, the administration has attempted to reinterpret the terms to allow testing of Reagan's "Star Wars" missile defense system. Senate Democrats have insisted that the stricter interpretation understood by the Senate at the time of ratification be honored.

The new medium-range missile pact, signed by Reagan and Gorbachev in Washington last December, requires the destruction of some 2,611 missiles with the capability of delivering nearly 4,000 nuclear warheads.

All missiles with ranges between 300 and 3,400 miles - whether deployed on launchers or in storage - must be destroyed under the agreement.

Although the small band of opponents was buried beneath the massive support for the agreement, the Senate's five-month examination showed the pact to have flaws the administration had not realized when it submitted the treaty to the Senate last January.

Final debate was delayed several days while Secretary of State George P. Shultz returned to Geneva for a follow-up meeting with Soviet Foreign Minister Eduard A. Shevardnadze to resolve questions raised by the Senate Armed Services and Intelligence committees.

As a result, all efforts to amend the text of treaty were defeated.