Tarasyuk, a close aide to liberal opposition leader Viktor Yushchenko, who says he was robbed of victory in last Sunday's election by large-scale fraud, said on Thursday Russian spetsnaz commandos were near presidential buildings in Kiev.
Street protests are continuing in the Ukrainian capital after a disputed presidential election.
In Kiev, there was no evidence that such forces were being deployed though rumours suggesting that have circulated for days.
Election results, which gave Moscow-backed Prime Minister Viktor Yanukovich victory, have been called fraudulent by independent observers and rejected by the United States. Hundreds of thousands of protesters have taken to the streets.
"Some 800 Russian spetsnaz troops, perhaps as much as 1000, are in Ukraine armed to their teeth," Tarasyuk, a former foreign minister, told Poland's parliament, which has backed Yushchenko in what deputies see as Ukraine's fight for democracy and ties with the West.
"Spetsnaz is preparing to meet the Ukrainian people in the heart of Ukraine, in its capital ... . Our deputies saw first hand spetsnaz troops at the presidential administrative building, armed with machine guns and live ammunition."