MOSCOW, NOV. 2 -- At least six people were killed today in ethnic clashes in a predominantly Russian-speaking region that has declared its independence from the rest of the Soviet Union's southwestern republic of Moldavia.

The official Soviet news agency Tass said the clashes occurred near Dubossary on the Dniester River, which separates the ethnically Romanian and Russian regions of Moldavia. It said police opened fire after armed pro-Romanian nationalists, who want to merge Moldavia with Romania, crossed a river bridge and began fighting with Russians who have proclaimed their own republic inside Moldavia.

But an official of Moldavia's Interior Ministry said the firing was done by Interior Ministry troops after 100 or more Russians set up roadblocks on the bridge and began throwing stones at the troops. The Russians had seized a government building in Dubossary, the official said.

President Mikhail Gorbachev told Soviet television news that he was "very alarmed" by the situation in Moldavia, a small republic of 4.2 million people annexed from Romania in World War II. He said there were signs that the situation could deteriorate and "there might be great trouble."

"The key thing now is to stop what's going on because clashes have already occurred," said the Soviet leader, who is scheduled to meet on Saturday with Moldavian President Mircea Snegur to discuss how the Kremlin can help stabilize the situation.

Moldavian authorities announced today that a state of emergency imposed last week on a southern region inhabited by an ethnic Turkish minority known as the Gagauz had been extended to include the Russian-speaking areas in eastern Moldavia. Thousands of armed pro-Romanian nationalists have massed at the borders of both the Gagauz and Russian regions in an attempt to prevent these areas from seceding from Moldavia.

Leningrad television later reported that the death toll in Dubossary, a town of about 20,000 that is one of several in the region inhabited largely by Russian speakers, had risen to eight. At least 30 people were wounded in the violence, according to Tass.

This year alone, at least 500 people have been killed in clashes in Central Asia and the southern Transcaucasus region as a result of the flareup of ethnic tensions that has accompanied the general relaxation of political controls under Gorbachev. Today's clashes were the first major outbreak of violence in western regions of the Soviet Union.

The leading pro-Romanian nationalist organization, the Popular Front, has called for the republic's "immediate and unconditional" secession from the Soviet Union and reunification with Romania. But the Russian-speaking minority in Moldavia, which numbers roughly a third of the total population, declared its independence from the rest of the republic in June in response to the resurgence of Romanian nationalism. The Russians proclaimed a new Trans-Dniestran republic, with its capital in Tiraspol, and elections for a local legislature have been called for Nov. 25.

Russian mayors in Tiraspol and other predominantly Russian cities such as Dubossary refused to fly the Romanian tricolor from public buildings, continuing to display the Soviet hammer-and-sickle flag. They also declared invalid new Moldavian laws requiring all state employees to learn Romanian.

The Russians have made common cause with the Gagauz, an ethnic Turkish nation with long Christian traditions, in their struggle against the ethnic Romanian majority.

In another development that could further strain relations between authorities in Moscow and the republics, Gorbachev announced plans tonight to exercise tighter central control over Soviet hard-currency revenues. A presidential decree said that 40 percent of next year's export revenues would be used to repay the country's $60 billion foreign debt.

Gorbachev's decree flatly contradicts recent moves by the giant Russian republic to gain control over its vast reserves of oil, gold and diamonds. A committee with representatives of the republics will be set up to allocate 90 percent of all remaining foreign exchange revenues, effectively depriving Russia of its right to sell its raw materials on world markets.