President Boris Yeltsin, who signed the accord with his Belarusian counterpart Alexander Lukashenko, stressed the two Slavic nations will remain sovereign and separate.
"We'll not transform our community into a unified state for now, but a union of two states," Yeltsin said at the signing ceremony in a gilded Kremlin hall.
No other ex-Soviet republics have shown a real desire for reunification since the Soviet Union collapsed in 1991, creating 15 separate states.
Although Russian hard-liners hailed the treaty as a step toward the revival of a mighty state, Belarusian opponents expressed fear their country of 10 million people would lose its independence to much larger Russia.
Critics also argued that integrating the two struggling countries will only create more problems for both.
About 5,000 opponents of the accord tried to march to the Russian Embassy in the Belarusian capital, Minsk. Riot police confronted them, clubbing and hitting the marchers when the protesters threw stones at them.
The clash was the worst violence in Belarus in months. Russian news broadcasts showed policemen beating women with truncheons and an officer swinging his boot to hit an injured protester who was lying on the pavement.
Opposition and human rights activists said about 200 people were beaten and detained. The government confirmed 70 people were detained and three policemen were injured, but declined to provide more information.
In Moscow, small groups of opponents staged protests, warning against any alliance with Lukashenko, an authoritarian leader nostalgic for the old Soviet Union. He has proposed - and Moscow has rejected - a full merger.
Under yesterday's treaty, the two countries plan to coordinate economic reforms and military activities, create joint energy and transportation systems and possibly introduce a common currency. A Supreme Council, including top leaders from both countries, is to outline joint policies.
Although the treaty calls for consultations on a wide variety of policies, it proposes little action. Russia and Belarus have removed customs barriers but otherwise have done little toward integration since forming a "community" a year ago.
The Russian news media and liberal politicians say that Belarus, which has yet to reform its Soviet-era economy, could drag cash-strapped Russia down.
Lukashenko responded angrily to that yesterday, saying, "It will never be true that our small country will be a heavy burden to Russia."
Neither government presented the treaty to the public before the signing ceremony, and critics say its secrecy was also cause for concern.
"Is it a czar or a president who governs Russia?" asked Alexander Lebed, Russia's popular former security chief. "If it's the president, he should have informed the people about terms of unification with Belarus."
Lebed said he agreed with integration, but not for such a hastily prepared union.
"The construction of a house should begin with laying a foundation. In our case, the construction has started from the roof. Such houses do not last for long," he said, according to the Interfax news agency.
Yeltsin, apparently heeding critics, signed an agreement that contained only nine clauses, a much shorter document than originally planned. However, the leaders also initialed a more wide-ranging charter that will be submitted for public discussion over the next six weeks.
Elated Russian hard-liners praised the treaty as a step toward a return of a powerful state.
"Congratulations! We live in a union again," said Communist Party head Gennady Zyuganov. "It's a good thing that Belarus is first. The union once again stretches from Brest to Vladivostok."
AP PHOTO
Police officers beat a demonstrator whose head is covered with a nationalist flag in Minsk, Belarus yesterday. Police officers in full riot gear confronted about 5,000 demonstrators protesting the reintegration treaty signed yesterday.