PARIS, NOV. 19 -- Soviet President Mikhail Gorbachev warned other European countries today not to encourage separatist trends in the Soviet Union if they want to preserve the remarkable breakthrough that has occurred in East-West relations over the past year.

The Soviet leader's blunt statement to the 34-nation European security conference, which opened here today, reflected a recent deterioration in relations between the Kremlin and outlying Soviet republics. At least five of the 15 republics have sent their own representatives to the meeting in Paris, saying that they do not recognize the right of the Soviet delegation to represent them.

While praising the reunification of Germany and the end of Soviet domination over Eastern Europe as positive developments, Gorbachev in effect told the West that any attempt to dismantle the former Russian empire could be extremely dangerous. He said that "militant nationalism and reckless separatism" could lead to the "Balkanization" or even the "Lebanization" of entire regions, undermining East-West detente.

"Claims to territorial changes would seem particularly unacceptable, for once the process has begun it may produce a destructive snowball effect that would throw Europe back to a situation it knows so well from its history," Gorbachev said.

The Soviet leader's speech in the grand hall of the Kleber conference hall here was piped into a small, stuffy room in the same building that is being used as a holding area for the representatives of Soviet republics demanding their own seats at the conference table. Several officials from the republics complained that they were being barred from taking part in the new Europe because the West is placing all its bets on Gorbachev.

"What's happening here is clear. Gorbachev is effectively giving the West a green light to do what it wants in Iraq. In return, he is getting a carte blanche to do what he wants inside the Soviet Union," complained Galastyan Ambartsum, national security adviser to Soviet Armenia's newly elected nationalist president.

"Everybody is speaking about the end of the Cold War, but it has not ended for us," complained Marju Lauristin, deputy speaker of the Estonian parliament. "Look, they've even put us in a room with metal bars on all of the windows. We're not allowed to go anywhere else in the building. So this is the new Europe."

The Baltic delegates, who consider their countries to have been illegally annexed by the Soviet Union in World War II, received a psychological boost when Czechoslovakia and Denmark called for them to be granted "observer status" at the conference. But such a step is unlikely since all decisions are traditionally made by consensus at the conference.

In an attempt to avoid embarrassment in Paris, the Kremlin included representatives of several Soviet republics in its official delegation. Thus, while Armenia's former Communist foreign minister walked in the front door with Gorbachev, Ambartsum was reduced to trying to find a pay phone on the nearby Champs-Elysees to complain about it to his government in Yerevan, the Armenian capital.

Asked about the controversy, presidential press spokesman Vitaly Ignatienko replied, "We have an Armenian in our delegation. At least, he looks like an Armenian. This is not a serious question."

Meanwhile, Leonid Kravchuk, president of the Ukraine and a career Communist apparatchik, called on the summiteers in a letter distributed here to allow the Ukraine to participate in the meeting as a "founding member state of the United Nations."

The French organizers of the meeting have used an elaborate system of colored badges to rank delegates and would-be delegates. Thus San Marino, with a population of about 25,000, is entitled to a full motorcade and presidential guard snapping to attention, while the Ukraine, with a population of 53 million, gets a simple visitor's pass that has to be renewed twice daily.

The foreign ministers of the three Baltic states, who were barred from participating in the conference, had been invited as "guests" by the French authorities.