Delegates from Europe, the United States and Canada reached agreement today on the ground rules for reviewing East-West detente and human rights in Belgrade later this year.

The Breakthrough after seven weeks of hard bargaining followed a concession by the Soviet Union; which had insisted on a firm cutoff date for the talks. Today it proposed an ambiguous formula that was acceptable to Western delegations. The agreement is subject to approval by the 35 governments that signed the Helsinki agreement on East-West debate in 1975.

Announcing the agreement to reporters, the chief U.S. delegate, Albert Sherer, said that it met all the main objectives sought by the United States.

The meeting is to open Oct. 4 and complete its work by Dec. 22. If necessary, it can continue from mid-January to the middle of February to draft a concluding document.

Western delegates say that a commitment to produce a final document leaves open the possibility of the meeting's continuing beyond the middle of February if there is a deadlock.

Disputes over the cutoff date have held up agreement at the present talks., The Soviet Union's demand for a fixed time limit were opposed by Western delegations on the ground that it could lead to a filibuster.

"This will insure a thorough review of the implementation of all parts of the 1975 Helsinki agreement," Sherer said after emerging from a final four-hour drafting session involving representatives of the 35 signatories of the declaration.

Other American delegates said they thought final agreement would allow plenty of time for discussion of progress on the controversial human-rights pledges of the Helsinki declaration. Several members of Congress already have indicated their intention of coming to Belgrade to debate human rights directly with the Soviet Union.

Under the agreement, specialist committees are to examine compliance with various aspects of the Helsinki declaration. A separate commitee devoted to humanitarian questions is to meet at least 40 times.

The latest concessions by the Soviet Union are largely a result of a last-minute stand by Britain and a small group of Western countries, including the Netherlands and Portugal. Other members of the NATO alliance, such as West Germany and Norway, favored accepting a compromise package at the end of last week.

For its part, the Soviet Union has not done too badly in the present talks. It has succeeded in imposing an eight-week times limit on the work of the committees and has avoided the prospect of entirely open-ended negotiation on human rights.