* Rousseff says has committed no crime to shorten term

* Odebrecht says executives to seek plea bargain deals

* Rouseff govt fate rests with main coalition party (Adds Odebrecht to cooperate with prosecutors, wiretap ruling)

By Anthony Boadle

BRASILIA, March 22 (Reuters) - President Dilma Rousseff said on Tuesday she will not resign in Brazil's worst political crisis in two decades, calling an opposition move to impeach her a "coup d'etat" against democratic rule because she had committed no crime.

A corruption scandal that has reached her inner circle threatened to implicate more people after the country's largest engineering firm Odebrecht decided to cooperate with prosecutors investigating a huge political bribery scheme.

"I will never resign under any circumstances," the embattled president said in a speech to legal experts. "I have committed no crime that would warrant shortening my term."

Rousseff called on Brazil's Supreme Court to remain impartial in the crisis that has threatened to topple her government as opponents seek her impeachment in Congress.

Opposition parties have launched impeachment proceedings against Rousseff for allegedly manipulating government accounts to allow her government to spend more in the run-up to her 2014 re-election. The president could be suspended as soon as May if her supporters do not block impeachment in the lower house.

Recent corruption allegations and huge anti-government street protests have raised the odds of Rousseff being impeached, ending 13 years of leftist Workers' Party rule.

The Petrobras graft investigation has implicated dozens of politicians in Rousseff's coalition and led to the jailing of scores of executives in top engineering firms such as Odebrecht. Following police raids on company offices on Tuesday, Odebrecht said in a statement its executives targeted in the corruption probe will seek plea bargain deals with prosecutors.

With her popularity at rock bottom due to the snowballing scandal and the worst recession in a generation, the political survival of Brazil's first female president depends largely on her main coalition partner, the centrist Brazilian Democratic Movement Party (PMDB).

Growing numbers of lawmakers in the fractious PMDB want the party to leave her government, a decision that could be taken at a March 29 executive committee meeting. The party may hold the deciding votes on impeachment, which would put Vice President Michel Temer, leader of the PMDB, in the presidential seat.

Party officials have denied Brazilian media reports that Temer is already preparing a post-Rousseff government and has begun talks with opposition leaders to secure their backing.