* Election authority, private polls show Correa ahead
* Opponents fear president amassing excessive powers
* Controversial proposals give Correa power over judiciary (Adds details throughout)
By Eduardo Garcia and Alexandra Valencia
QUITO, May 7 (Reuters) - Ecuador's President Rafael Correa was on course for a comfortable victory in Saturday's referendum on judicial and media reforms opponents say are a power grab undermining the Andean nation's democracy.
"The Ecuadorean people have triumphed," said Correa, claiming victory shortly after polls closed.
The national election council said its first "quick" sample count showed him ahead on all 10 questions by a range of 51-57 percent in what was essentially a vote of confidence on the charismatic Correa, 48, who has governed since 2007.
The vote looks sure to strengthen the popular but domineering Correa's political grip on the OPEC member nation and put him in a good position for a re-election bid in 2013.
Correa supporters danced and sang in jubilation at party headquarters into the night on Saturday.
Two private polls of voters during Saturday's ballot showed all the reforms passing by about 57 and 60 percent, respectively -- a wider margin than the first official data.
Some opposition leaders quickly acknowledged defeat.
"It's indisputable he won," the president's brother and critic Fabricio Correa told Reuters. "But it wasn't a thrashing like they said. ... It's time for meditation not triumphalism."
Full coverage of Ecuador's referendum [ID:nECUADOR]
Political risks in Ecuador [ID:nRISKEC]
Correa has maintained high popularity among the poor and lower-middle classes thanks to big spending on schools, roads and hospitals. Rivals accuse him of an autocratic streak akin to his ally Hugo Chavez, the president of Venezuela, and fear he could use the reforms to persecute opponents.
"We're giving Correa too much power, but it doesn't matter. Someone has to tidy up this mess," said "Yes" voter Cesar Acosta, 67, echoing Correa's original justification for the vote to root out graft and inefficiency in courts.
The most controversial two proposals set the foundations for a new justice system in which Correa will have more direct say over appointments. He argues that corrupt judges have to go so police can better fight crime in the nation of 14 million.
"We have to make big changes in the next 18 months. We're going to face the opposition of mafias within the judiciary," Correa told state TV after claiming victory.
Analysts say Correa's ability to control a new three-person panel charged with overhauling the justice system and appointing judges to the Supreme Court and lower tribunals will further concentrate power in the executive and limit checks.
"I think he's tired of the constitution," former presidential aide turned critic Alberto Acosta told Reuters.
Other reforms include more controls on media, with whom Correa has been dueling regularly, and a ban on bullfights to the consternation of fans of the sport.
The strongest leader in decades in a country notorious for political instability, Correa bristles at claims he is taking Ecuador down an autocratic route. "We are here to change our Ecuador in peace and democracy," he said.
Not surprisingly, Chavez was the first foreign leader to congratulate his "brother" for a "great victory" in a Twitter message and official Venezuelan government communique.
Ecuadoreans appeared to vote en masse, especially given the threat of a $25 fine for not casting a ballot. Local TV showed voters walking through muddy roads in the Amazon area, wearing indigenous outfits in an Andean province and lining up in front of polling stations in the Galapagos Islands.
The flamboyant and voluble Correa forms part of a regional alliance of leftist Latin American presidents that includes Evo Morales in Bolivia as well as Chavez.
They are fervent critics of U.S. "imperialism" and have sought to boost state revenues from their country's energy resources to spend on social projects.
Correa is likely to feel emboldened, but his victory will also fuel criticism he wants to install communist-style rule in Ecuador, and that could cost him support among educated voters in Quito and Guayaquil, Ecuador's biggest urban areas.
Victory should allow Correa to rein in dissent in the ruling Alianza Pais movement and better control parliament.
Analysts do not expect Correa to take any dramatic new measures against foreign investors, whom he has already largely strong-armed into deals more favorable to the state.
"In the oil sector, the government has already extracted significant benefits. ... (It) will offer mining companies relatively generous terms in its initial contracts in order to attract foreign investment to help develop the nascent mining sector," Eurasia Group consultancy said.
Correa's tough stance toward the media has fueled fears that reforms to limit media ownership and hold journalists "responsible" for stories will let him silence criticism.
After the vote, Correa again lashed the "corrupt press" he said tried constantly to smear him. "There's no problem with opposition, but what they can't do is misinform and lie." (Additional reporting by Jose Llangari and Santiago Silva in Quito, Mica Rosenberg in Bogota; Writing by Andrew Cawthorne; Editing by Todd Eastham)