The cutoff came just hours after Gazprom's chairman, Dmitry Medvedev, won the Russian presidential election with the support of his mentor President Vladimir Putin.
Putin and Medvedev have increased the Kremlin's control over Gazprom and used it to help energy-rich Russia expand its economic and political influence abroad. The action seemed to suggest that Medvedev will continue Russia's tough stance toward Ukraine, which is seeking closer ties with the West, including NATO membership.
Supplies to Ukraine were reduced by 25 percent, or 30 billion cubic meters, said Valentyn Zemlyansky, a spokesman for Ukraine's Naftogaz.
Gazprom almost immediately accused Ukraine of siphoning gas from the company's pipelines, which ship gas across its territory to other European countries. The action followed a threat by Gazprom to reduce supplies by 10 a.m. Moscow time (0700 GMT) if the Ukrainian government did not approve documents on debt payment and future deliveries.
"Gazprom is a reliable supplier of energy resources, but we cannot and should not deliver gas without payment," Gazprom spokesman Sergei Kuprianov said in a statement Monday.
It said supplies to European consumers would be unchanged.
While Russia has threatened to sever supplies to Ukraine several times in recent years, the only previous time it did so was on New Year's Day 2006, when the cutoff affected some European consumers. Much of the Russian gas consumed in Western Europe comes in pipelines that cross Ukraine.
Ukrainian President Viktor Yushchenko and Putin reached an agreement last month to avert another threatened cutoff. Yushchenko insisted last week that Ukraine had paid off its debt - but Gazprom said the payments only covered 2007 supplies, not debt still outstanding from previous years.
Naftogaz officials were expected in Moscow this week to continue negotiations over the debt payment, Naftogaz spokesman Ilya Savvin said.
Ukraine imports gas both from Central Asia and from Russia, all of it arriving in Gazprom-controlled pipelines.
Gazprom says that about 25 percent of Ukraine's gas imports are Russian-origin; that is the portion Gazprom had said it would cut off Monday if the documents were not signed.
The dispute underlines Ukraine's dependence on Russia even as Yushchenko tries to move the country closer to the West.