* Thousands pour into Kiev for fourth anti-Yanukovich rally
* EU suspends work on trade deal with Ukraine
* Opposition warns against Moscow "sell-out"
* U.S. Senator McCain: "Your destiny is with Europe"
By Richard Balmforth and Natalia Zinets
KIEV, Dec 15 (Reuters) - Thousands rallied on Sunday against Ukrainian President Viktor Yanukovich just days before he heads for a meeting in the Kremlin and opposition leaders told him not to bother coming back if he "sells out" Ukraine.
Minutes before the rally, EU enlargement chief Stefan Fuele said on Twitter he had told Ukraine he was suspending work on a trade and political deal, which should have been signed two weeks ago, saying Kiev's arguments to improve terms had "no grounds in reality".
Fuele's words suggested the European Union has lost patience with Kiev's demands for financial aid and was irritated at the way the bloc was being forced to take part in a 'bidding war' with Russia over Ukraine.
The focus was now on a visit Yanukovich is due to make to Moscow next Tuesday to tie up trade agreements with the Kremlin to help the distressed Ukrainian economy, but which the opposition fears will slam the door on integration with the European mainstream.
In particular, they fear he may take the first steps towards joining a Moscow-led customs union, together with Belarus and Kazakhstan, which they see as an attempt by Putin to re-create the Soviet Union.
"He might as well stay in Moscow and not come back to Kiev if a customs union agreement is signed," declared former economy minister Arseny Yatsenyuk, one of the opposition leaders. "We'll give him a really warm welcome if he sells out Ukraine."
"The Kremlin wants to take its revenge on Ukraine, divide Ukraine and drown it in blood," said far-right nationalist leader Oleh Tyahnybok. "We forbid this president to sign anything in Moscow that contradicts the interests of the Ukrainian state."
Yanukovich may be attempting to keep the attention of both Moscow and Brussels to strike as good a deal as possible to handle its huge debt and outstanding gas payments to Moscow. But it is a hazardous manoeuvre running the risk of alienating both parties.
Opposition leaders called for another mass rally on Tuesday to monitor Yanukovich's trip to Moscow and any deals made there. 'DESTINY IN EUROPE'
Earlier, U.S. Senator John McCain galvanised the 200,000 or so people on Kiev's Independence Square, telling them their destiny lay in Europe.
"We are here to support your just cause, the sovereign right of Ukraine to determine its own destiny freely and independently. And the destiny you seek lies in Europe," said McCain, a leading Republican voice on U.S. foreign policy.
Street protests erupted after Yanukovich's decision on Nov. 21 to walk away from the agreement with the EU, after years of careful preparation, and turn to Moscow, Kiev's Soviet-era overlord, for aid to save Ukraine's economy.
Yanukovich's policy swerve, while backed by many in Russian-speaking east Ukraine which is his powerbase, sparked huge disappointment and anger in western and central areas where people see Europe as their proper place.
The presence of McCain at the anti-government rally after a weeks-long stand-off between demonstrators and the authorities further highlighted the geo-political East-West tug-of-war which Ukraine is once again at the centre of.
The Republican senator is the latest of a string of European and American dignitaries to tour the sprawling protest camp set up behind barricades of benches, metal barriers, supermarket trollies and wire netting on the square - known locally as the 'maidan'.
Russian Prime Minister Dmitry Medvedev has accused Western politicians of "crude" meddling in Ukraine's affairs.
The movement began as a low-level pro-EU protest.
But after a police crackdown on a group of mainly students and a later face-off between police and protesters last Wednesday, it has broadened into an outpouring of anger against perceived sleaze and corruption in the country Yanukovich has led for four years.
BATTLE FOR SOUL
Protesters characterise it as a battle for Ukraine's soul.
McCain, who met opposition leaders - the former boxing champion Vitaly Klitchko as well as Yatsenyuk and Tyahnybok - said: "We ... want to make it clear to Russia and Vladimir Putin that interference in the affairs of Ukraine is not acceptable to the United States."
Speaking to journalists after addressing crowds, he said it was disturbing to hear that the EU may be suspending talks with Ukraine on the trade and political agreement.
Yanukovich, whose allies hold a majority in parliament and who still appears to command loyalty in the security forces, seems likely to hang on to power despite the strength of peaceful rallies and opposition calls for early elections.
But much may depend now on what sort of deal he can cut with Putin next Tuesday on cheaper gas and credits, how well he can present it to his people and how quickly any help will trickle down to Ukraine's creaking economy.
But any step by Yanukovich towards the Moscow-led customs union, which the opposition sees as a return to the Soviet Union, will be a dangerous one for him to take.
Whatever the outcome, his popularity has suffered hugely from the crisis, the opposition has been re-energised, the faith of key stakeholders such as the oligarchs has been shaken and he can no longer assume re-election in 2015 is in the bag.
Klitschko's UDAR party called on Sunday for the dismissal of Andriy Kluyev, one of Yanukovich's closest security aides, whom the opposition says was behind past attempts to break up the protests by force.
On Sunday, the crowds gathered on Independence Square were smaller than a week before but no less determined.
"I am here against the criminal authorities, joining Europe is a secondary goal," said Oleksander Vdovin, 25, an engineer in Kiev wrapped in a Ukrainian flag.
Yanukovich's supporters have also staged rival rallies nearby on Saturday and Sunday.
"We are here because an effort to destabilise the country has begun. I voted for the president, I'm here to back him," said Nikolai, 61, who works in the southern Ukrainian port of Kherson.