Syrian President Hafez Assad today sharply criticized U.S. mediation efforts in the Middle East as President Reagan's new special envoy, Robert C. McFarlane, began a swing through the region's capitals in search of ways to break the deadlock over foreign troop withdrawals from Lebanon.

With McFarlane spending his first day in the region in talks with Lebanese government leaders in Beirut, the Syrian president today reiterated his opposition to all previous U.S. peace plans and proposals in the area--from the 1978 Camp David accords to the recently negotiated Israeli-Lebanese agreement--and challenged the fairness of any U.S. mediation in the light of Washington's close political and military ties to Israel.

The president's remarks, delivered in a speech marking Syria's armed forces day today, reinforced pessimism among foreign and Arab analysts here over the ultimate outcome of McFarlane's mission to breathe new life into the stalled U.S. peace-making efforts in the Middle East.

"How could the U.S. be a fair mediator between any Arab party and Israel when it has been encouraging and fully backing Israel's continuous aggression against the Arabs?" Assad asked.

The president said, "Through the state of disorder deliberately created by the U.S. in the Arab area the U.S. has claimed itself as a mediator and arbiter not only between the Arabs and Israel but between the Arabs themselves.

"This mediation cannot be accepted unless we accept that the enemy can be a fair judge. This is only accepted by those who give themselves up to the United States"--something Assad made clear he would never do.

Assad strongly reiterated his opposition to the U.S.-brokered Israeli-Lebanese agreement that McFarlane hopes to get the Syrian government to accept. The agreement, which provides for Israeli troop withdrawal from Lebanon, is conditional on Syria and the Palestine Liberation Organization agreeing to withdraw their troops.

Claiming the agreement was part of a "Zionist-American hegemonistic plan" in the Middle East, Assad termed it "worse than the Camp David accords" that his government steadfastly opposes.

The president's speech seemed sure to dampen whatever optimism there might have been in Washington after Syria agreed to see McFarlane during his six-nation tour of the area. In May the Syrian government refused to have any further dealings with U.S. special Middle East envoy Philip Habib--McFarlane's predecessor--thus effectively ending Habib's usefulness as a mediator.

Diplomatic sources here say it was Assad's refusal to reconsider this ban during talks with U.S. Secretary of State George P. Shultz here early last month which set in motion Habib's retirement nine days ago and the appointment of McFarlane, Reagan's deputy national security adviser, as his replacement.

Though McFarlane's first mission to the Middle East has been surrounded by speculation that he is bringing "new ideas" to the region to try to break the diplomatic stalemate, Western and Arab diplomats here questioned whether McFarlane could have any proposals that would alter Assad's position.

When Shultz met Assad for five hours July 6 during a hastily scheduled swing through the region, the Syrian president informed Shultz that the Syrian stand on the Lebanese agreement was "final and irrevocable."

According to diplomats here most of the five hours of that meeting were spent with Assad talking and the secretary of state listening. Assad apparently gave Shultz a point-by-point critique of the agreement, explaining just why he could never accept the accord that resulted from months of difficult U.S. mediation between Israel and Lebanon.

Assad opposes the accord for two reasons. He considers the agreement a security threat to Syria because Israel would control a buffer zone in southern Lebanon within artillery range of Damascus. Also, the agreement imposes conditions on the Lebanese government which Assad believes are outright violations of Lebanon's sovereignty and independence that would reduce Lebanon to the status of a vassal state.

Western diplomats here believe the only thing that would convince Assad to withdraw his troops from Lebanon would be the scrapping of the current Israeli-Lebanese accord and the negotiation of a new one. Under such an agreement, Syria would demand security concessions in Lebanon on a par with Israel's.

However, as one western diplomat said, "That, unfortunately, is just not going to happen."

Syria's position now is that it will not even consider withdrawing its estimated 40,000-man military force in northeastern Lebanon until Israel withdraws all of its troops from southern Lebanon "unconditionally."

The Syrian position is based on the claim that Syrian forces are in Lebanon legally, at the request of the legal government of Lebanon and the Arab League, while Israel's are there illegally as a result of their military invasion of the country last summer.

The timing of McFarlane's arrival in Damascus was not known as the State Department has kept his itinerary through the region secret.