* Russia, Ukraine deny deal on customs union, cheaper gas
* Yanukovich, Putin held surprise talks on Friday
* Opposition seeks mass rally on Sunday
KIEV, Dec 7 (Reuters) - Ukraine's opposition accused President Viktor Yanukovich on Saturday of betraying national interests at unannounced talks with Russia's Vladimir Putin, though both countries said nothing had been agreed.
Both governments denied reports they had agreed a deal on cheaper Russian gas and for Ukraine to join a Moscow-led customs union after Yanukovich last month spurned a trade pact with the European Union.
But the surprise meeting in the Black Sea resort of Sochi on Friday - and speculation that a deal had been struck - risks fuelling public anger in Ukraine, where thousands of protesters are occupying parts of the capital Kiev in a dangerous stand-off with security forces.
"Why did he need this agreement in Sochi? It is not in the interests of the fate of the country," Arseny Yatsenyuk, one of three main leaders of the opposition protests, told crowds gathered on Independence Square under driving snow.
"He is only interested in his own personal fate," Yatsenyuk said. "He needs money for the survival of his regime. We demand that he and (Prime Minister Mykola) Azarov come out and tell us what's happening."
Yanukovich's Nov. 21 decision to ditch a deal with the EU on trade ties and integration has thrown the country of 46 million people into turmoil, triggering protests on a scale not seen since the 2004-05 Orange Revolution.
Faced with $17 billion in gas bills and debt repayments next year, Ukraine is instead seeking cheaper gas imports and possibly credits from Russia to stave off a looming balance of payments crisis. Those bills are about as much as the heavily-depleted reserves now held by Ukraine's central bank.
Analysts say Putin is trying to lure Kiev into a regional Customs Union, led by Moscow and including ex-Soviet republics Belarus and Kazakhstan.
"IT'S ALL RUMOURS"
The EU pact would have heralded a historic shift westwards, but the opposition protesters now accuse the government of turning back the clock by forging closer ties with Ukraine's old Soviet master.
Dmitry Peskov, a spokesman for Putin, said the two governments would meet on Dec. 17 in Moscow, but denied any deals had been struck.
"No final agreement has been reached," Peskov told reporters. "Talks on all these issues will be continued on the level of experts in the near future."
Both Peskov and Yanukovich's press service denied the presidents had discussed Ukraine's membership of the Moscow-led customs union.
"No documents during the meeting were signed," Yanukovich's office said in a statement on his official website. "Special attention was paid to cooperation between Ukraine and Russia on the energy sector."
"It's all rumours," a spokesman for Prime Minister Azarov told Reuters.
The speculation, however, risks stirring more turmoil in Kiev, where opposition leaders have called another mass rally for Sunday.
Police have threatened to eject protesters occupying public buildings, including Kiev's City Hall. A police crackdown last Saturday triggered outrage in Kiev and helped bring out some 350,000 the following day.
With the first snowfall of winter, protesters on Independence Square are digging in with tents and supplies of food and clothing. Men in construction helmets control passage in and out of the square through barricades of plywood, park benches and the dismembered branches of a giant artificial Christmas tree.
The scene recalls the Orange Revolution, when Ukrainians camped out for months through winter to overturn a fraudulent presidential election won by Yanukovich. He took power in 2010.
Interfax news agency reported that protesters planned to picket three television stations in Kiev and that police had posted forces outside the offices of the public broadcaster, citing opposition threats to take them over.
(Additional reporting by Pavel Polityuk in Kiev and Alexei Anishchuk in Moscow; Writing by Matt Robinson; Editing by Gareth Jones)