* Cause of failure under investigation
* Blackout lasts nearly five hours in Havana (Updates with new government announcement)
HAVANA, Sept 10 A failure in a main transmission line in central Cuba caused a blackout that cut electricity for hours to more than half the country on Sunday night, but now power has been mostly restored, the government said on Monday.
The Ministry of Basic Industry said in a statement there was an "interruption" in a 220,000-volt line between the cities of Ciego de Avila and Santa Clara, which caused a power outage that stretched more than 450 miles (725 km) from Camaguey in the southeast to the westernmost province of Pinar del Rio and included the capital Havana.
The blackout partially affected three provinces, but was total from Villa Clara, about 160 miles (257 km) east of Havana, west to Pinar del Rio, an area that includes the capital city, according to an update read on Cuban television.
It said the national electricity system was "normalized" at 2:30 a.m., using emergency generators and power from 13 "isolated" systems, but warned there could be more blackouts during peak usage hours in the western provinces until three major generating plants are brought back on line on Tuesday.
The cause of the transmission line failure was being investigated, but apparently the three plants shut down, which usually happens when a power system suffers a major problem.
The ministry did not say how many people were affected. Cuba has a population of 11 million, 2.2 million of them in Havana.
The power went out just after 8 p.m. on Sunday and was restored in some of the affected area within a couple of hours, but the outage lasted five hours or more in Havana where off-duty officers were called in to direct traffic in the dark streets with no functioning traffic signals.
Blackouts are not uncommon in Cuba due to its aging electrical system.
Hundreds of emergency generators were brought in starting in 2004 after a hurricane knocked down major transmission lines and left much of the country without power.
This blackout was more extensive than most in recent times and reminded some of the so-called "Special Period" in the 1990s when the country faced severe energy shortages following the collapse of the Soviet Union, its top benefactor.
Cubans said they did what they did during the long, frequent outages of those days - close up shop if they were in a retail establishment or just find a cool place and go to sleep.
"We stopped sales, collected the goods and put up a guard to avoid any crime," said restaurant worker Rodolfo Garcia.
Dianelys Carrera, an employee at a Havana convention center, said, "I opened the door (to my house), laid down on the floor and went to sleep."
Cuba is heavily dependent on Venezuela, whose President Hugo Chavez provides two-thirds of the country's oil under highly favorable terms in an oil-for-services deal. (Reporting By Jeff Franks and Nelson Acosta; editing by W Simon and Mohammad Zargham)