Leonid Brezhnev arrived here today displaying a new title -- President of the Soviet Union --and a strong determination to maintain detente with Western Europe despite the current deep chill in American-Soviet relations.

Speaking slowly and at times indistinctly in a brief statement to reporters on his arrival at Orly airport, the Soviet leader said that he and French President Valery Giscard d'Estaing would center their talks on "the most important and grave problem of our time, detente."

The three-day state visit to France --old Brezhnev in two years -- is seen by diplomatic experts here as a serious test by the Soviets and the French of a new global balance that could emerge if Washington and Moscow remain deadlocked on strategic arms negotiations, human rights and other subjects.

Meeting for almost three hours this afternoon in the 14th century chateau at Rambouillet, 30 miles southwest of Paris, the two leaders agreed on a special statement on detente that will be signed and made public Wednesday, French officials said.

French-Soviet relations have been a sensitive barometer for the general state of detente since the mid-1960s, when President Charles de Gaulle's determination to be independent of American strategic decision-making gave Paris privileged ties with Moscow and created the first impetus for East-West detente.

Those relations cooled in the recent years of superpower detente between the United States and the Soviet Union. Giscard's visit to Moscow in October 1975 ended in open discord.

Brezhnev's visit, postponed for a year, is the most concrete sign yet of a steady improvement in Paris-Moscow relations, apparent since the arrival of the Carter administration and a new set of international uncertainties.

Escorted by four French jet fighters, Brezhnev's Aeroflot jetliner arrived at Orly at mid-day under gray skies and 55-degree weather to a ceremonial reception that underscored his acquisition of the title of state president after the still unexplained ouster last week of Nikolai Podgorny.

Brezhnev walked heavily and carefully as he reviewed the honor guard with Giscard. His cheeks were puffy, he appeared to be wearing a hearing aid and French reporters who saw him on his trip here in December 1974 said he looked older and grayer than they had expected.

Brezhnev's health has been the subject of renewed speculation since reporters accompanying Secretary of State Cyprus Vance to Moscow in March reported that the Soviet leader looked tired or seriously ill during those meetings, which brought strategic arms negotiations to their present impasse.

Giscard listed detente, nuclear proliferation, disarmament and world tensions as the main topics of the talks and emphasized "the attachment France has to the policy of detente," which he called "the only alternative to confrontation and destruction."

Security surrounding the Soviet leader was extraordinarily stringent by French standards. It evidently was French standards. It evidently was heightened by two false bomb alerts in the airport area and an unconfirmed report that Soviet security men had asked the French to be on the lookout for a sniper.

A helicopter whirred over the 40-car cortege that sped with Brezhnev and Giscard from the airport directly to Rambouillet. A large, Soviet made limousine with black curtains drawn across its windows followed the car bearing the two leaders to the chateau, which is being protected by 3,000 gendarmes and riot policemen.

Brezhnev dined with his aides in the chateau this evening. He is to meet Giscard Tuesday for their second round of talks at the chateau, where he will have an opportunity to rest frequently during the day.

Brezhnev embroiled himself in France's tangled domestic politics late today by unexpectedly accepting an invitation to meet Paris Mayor Jaques Chirac Tuesday evening. Chirac, leader of the Gaullist Party, is Giscard's rival for leadership of the governing centrist-conservative coalition.

Africa is the only potential point of major discord in the visit, which both sides clearly want to succeed for their own reasons, according to diplomatic sources. France's help to the Zaire government earlier this year was explained by Giscard as an effort to stop Soviet-backed aggression there.

But those analysts say the desire for agreement on the detente will eclipse this and other differences that surfaced during Giscard's trip to Moscow in 1975.