(Repeats, includes dropped reference to paragraphs changed in preamble to text) (Updates with further comments from Ramirez, background on opposition effort, newspaper poll, paragraphs 12-16)

By Catherine Bremer

MEXICO CITY, Feb 21 (Reuters) - Mexican leftists, who paralyzed the capital with election protests in 2006, will go back to the streets this weekend to fight any attempt to let private capital into the oil sector.

But a top senator said on Thursday the leftist Party of the Democratic Revolution is willing to hold talks in Congress about an energy reform sought by President Felipe Calderon, possibly blunting the impact of street protests.

Hundreds of thousands of people went on marches in 2006 to support losing presidential candidate Andres Manuel Lopez Obrador's claims that Calderon stole the election by fraud.

Now Lopez Obrador hopes to rally his supporters outside the headquarters of the Pemex state oil monopoly on Sunday to protest at what he sees as plans to privatize the company.

But Sen. Graco Ramirez, the left's main voice on energy in the Senate upper house, said the debating chamber was still the best place to hammer out the future of Mexico's energy sector, which is grappling with declining output and reserves.

"We are up for the debate," said Ramirez, a secretary of the Senate energy committee which wants to agree a reform proposal backed by the three main political parties by April. He said his party would not boycott Congress energy debates to disrupt the reform, as sought by firebrand Lopez Obrador.

"We are not going on any parliamentary strike," he told Reuters in an interview.