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By Deisy Buitrago

CARACAS, Feb 7 (Reuters) - Tens of thousands of Venezuelans streamed through the streets of Caracas on Saturday to protest leftist President Hugo Chavez's second attempt to change the constitution to let him govern as long as he wins elections.

Opinion polls gave a slight lead to Chavez before a Feb. 15 vote on whether to allow the president and other politicians to run for re-election as many times as they like in South America's top oil exporter.

Voters rejected a similar proposal in 2007.

The march, under the slogan "No is no" and led by anti-government students and political parties that say Chavez will turn Venezuela into a version of communist Cuba, was the largest by the opposition in more than a year.

"This reform hides, as President Chavez himself has said, the start of what would be a country, a state with a Castro-communist system," said Manuel Rosales, a former opposition presidential candidate.

If he loses, Chavez would leave office in four years, but he does not rule out trying again to change the electoral law.

Another referendum defeat for him could embolden opponents and increase resistance to unpopular spending cuts or a currency devaluation that analysts say might result if oil income stays low.