Nicaraguan President Daniel Ortega, a former Marxist guerrilla leader, took a big early lead in Sunday's presidential election after heavy social spending won him strong support among the country's poor.

Nicaragua's electoral commission said preliminary results showed Ortega was ahead with 63.7 percent of the vote, far ahead of his closest rival, with votes counted from 18 percent of polling booths.

Ortega has overseen a period of economic progress in his five years in power, backed by financial aid from his socialist ally in Venezuela, President Hugo Chavez.

A former commander of the Sandinista rebel army that won power in a 1979 revolution and a Cold War adversary of the United States, Ortega has solidified his hold on Central America's poorest country with programs to improve health and education, microcredits and gifts of livestock.

There's no denying Ortega has done lots of good things, he's helped a lot of poor people, said Karla Flores, a 29-year-old mother of three from Masaya, southwest of the capital. He's got to keep up the good work.

Ortega had a big lead in opinion polls ahead of the vote over a conservative opposition whose two main candidates failed to unite against him.

He was able to run for re-election thanks to a 2009 ruling by the Supreme Court -- which his Sandinista party controls -- that did away with a ban on consecutive terms.

Backed by Venezuela, Ortega has reduced poverty in this largely agrarian nation and is credited with allowing the private sector to operate freely.

But Ortega is also blamed for undermining democratic institutions and some critics fear he aims to stay in power indefinitely like Chavez.

(Additional reporting by Alex Leff and Sean Mattson; Editing by Kieran Murray and Vicki Allen)