UPDATE 3-Sassou-Nguesso wins Congo Republic election

* Further seven years for Sassou-Nguesso

* AU says vote free, but rights group alleges fraud

(Adds turnout, background)

By Christian Tsoumou

BRAZZAVILLE, July 15 (Reuters) - President Denis Sassou-Nguesso has won seven more years in power in Congo Republic after taking 78 percent of the vote in Sunday's presidential election, the electoral commission said on Wednesday.

Observers from the African Union and the Economic Community of Central African States said the election, which opposition parties had boycotted, was free and transparent, but a human rights group said the vote was tainted by fraud.

"Congo salutes the victory of democracy. I am president of all the Congolese people," Sassou-Nguesso said, hailing victory at his campaign headquarters while his supporters honked car horns.

Sassou-Nguesso has been in and out of power since a 1979 coup in the country, Africa's fifth biggest oil producer. He won the last election in 2002 with almost 90 percent of the vote, a result dismissed by the opposition as a sham, after his main rivals were banned or withdrew.