* Return defies speculation Chavez too ill to travel
* Fireworks in street as supporters celebrate
* Questions remain on president's state of health
By Andrew Cawthorne and Deisy Buitrago
CARACAS, Feb 18 (Reuters) - Venezuelan President Hugo Chavez made a surprise return from Cuba on Monday more than two months after surgery for cancer that has jeopardized his 14-year rule of the South American OPEC member.
The 58-year-old socialist leader's homecoming in the middle of the night implies some medical improvement - at least enough to handle a flight of several hours - and will inspire supporters' hopes he could return to active rule.
Yet there was no new information on his state, nor images of his arrival, and aides say his condition remains "complex."
Chavez could simply be hoping to quieten political tensions in Venezuela and smooth a transition to Vice President Nicolas Maduro, whom he has urged voters to back should he have to stand down and a new presidential election is held.
"We have arrived back in the Venezuelan fatherland. Thanks, my God! Thanks, my beloved people! Here we will continue the treatment," Chavez said via Twitter after flying in.
After a six-hour operation in Cuba on Dec. 11, Chavez had not been seen or heard in public until photos were published of him on Friday.
There had been speculation Chavez was not well enough to travel despite wanting to return for continued treatment for the disease he was first diagnosed with in mid-2011.
"I remain attached to Christ and trusting in my nurses and doctors," Chavez also tweeted on Monday. "Onwards to victory forever! We will live and we will conquer!"
FIREWORKS MARK RETURN
But Maduro said Chavez flew in at about 2:30 a.m. 0700 GMT) from Havana and was in a military hospital in Caracas, where a crowd quickly gathered, chanting slogans and dancing.
Chavez's arrival thrilled supporters in the nation of 29 million people, where his common touch and welfare policies have made him an idol to many of the poor.
"It's fabulous news, the best thing possible," Chavez's cousin, Guillermo Frias, told Reuters from the president's rural birthplace in Barinas state. "Venezuela was waiting for him, everyone wants to see him. Welcome home! Thank God he's back!"
Fireworks were set off in some Caracas neighborhoods as news spread and celebrations began among "Chavistas."
Government ministers were jubilant with one singing "He's back, he's back!" live on state TV.
They asked Chavez's euphoric supporters to respect the peace of patients at the military hospital, near a hillside shanty-town, where a huge banner of the his face adorned one wall.
Soldiers guarded the installation, while supporters chanted "We are Chavez!" and "He's back, he's back!" At one point, medical staff came out and asked them to quieten down.
The December operation in Havana was his fourth for Chavez since cancer was first detected in his pelvic area in June 2011.
Officials have emphasized in recent days that Chavez's condition remains delicate. "It's a complex, difficult situation, but Chavez is battling and fighting for his life," Foreign Minister Elias Jaua said during the weekend.
On Friday, the government published photos showing Chavez lying in a hospital. Officials said he was breathing through a tracheal tube and struggling to speak.
Chavez's pre-dawn return was a typical surprise move for the former soldier whose rule has combined constant political theatrics with thundering anti-U.S. rhetoric, tough treatment of opponents and lavish spending of oil revenues on the poor.
Opponents have decried government secrecy over Chavez's condition, and some have called for a formal declaration that he is unfit to rule. That would trigger a new presidential election within 30 days, probably between Maduro and opposition leader Henrique Capriles.
Maduro, a 50-year-old former bus driver, is Chavez's preferred successor and would be favorite to win a close vote in such a scenario.
"Uncertainty over a possible presidential election remains intact, despite the president's return," Venezuelan political analyst Luis Vicente Leon said.
After winning re-election in October last year, and wrongly declaring himself cured, Chavez was unable to attend his own swearing-in ceremony in January. To the fury of his foes, Venezuela's Supreme Court ruled that he remained president and could be sworn in later.
That could now happen at the military hospital.
"Now the president is back, there can be no doubt about the democratic institutions working in Venezuela," Information Minister Ernesto Villegas said.
"There were some who dream of unseating Chavez and the revolution, but here we always said Chavez is the president elected and re-elected by will of the Venezuelan people."
Chavez's return eclipses national debate over a recent devaluation of the local currency. It has proved highly unpopular among Venezuelans and opposition parties have tried to present it as evidence of economic incompetence by the government.
His lengthy absence in Cuba had fuelled a long-held opposition accusation that Venezuela's government was being manipulated and directed from Havana. Former Cuban leader Fidel Castro is a political mentor and father figure to Chavez and Castro visited him regularly in the hospital.
"I'm pleased you have been able to return to the piece of ... soil you love so much and the fraternal people who give you so much support," Fidel Castro wrote to Chavez in a letter published by Cuba's government on Monday.
"You have learnt a lot about life, Hugo, in those tough days of suffering and sacrifice," he added, urging continued discretion over Chavez's condition to thwart "fascists" intent on toppling him.
Some 20 Venezuelan students have spent the past four days chained up close to the Cuban Embassy in Caracas in protest of what they see as interference from Havana in internal affairs.
Capriles welcomed Chavez back but pointedly said he hoped it would mean a return to order in government and attention to Venezuelans' daily problems.
Congressional leader Diosdado Cabello said Chavez was comfortably installed in the hospital. "We're fixing all the details there so he lacks absolutely nothing," he said.
One woman, identifying herself as a nurse at the hospital, told state TV that Chavez had arrived walking and without a wheelchair or visible tubes.