By Joseph Ax

NEW YORK, Nov 18 (Reuters) - Steven Donziger, the U.S. attorney accused of using bribery to secure a multibillion-dollar pollution judgment against Chevron Corp in Ecuador, took the witness stand for the first time on Monday and was immediately challenged on his claim that he was not in charge of the Ecuadorean case.

In a series of pointed questions, Chevron attorney Randy Mastro sought to undermine Donziger's claim that he did not lead the legal team in Ecuador but merely offered his advice.

"Isn't it a fact that you've described yourself as the lead lawyer?" Mastro asked.

"At times I have, yes," Donziger said.

Chevron filed a lawsuit in Manhattan federal court in 2011 claiming that Donziger used fraud to obtain an $18 billion judgment - later increased to $19 billion - for a group of villagers over the contamination of an oil field in northeastern Ecuador.

Last week, Ecuador's highest court cut the amount to $9.5 billion. Donziger has denied the bribery allegations.

The start of Donziger's testimony marked the most anticipated moment since the trial began last month, with more than 70 people in a packed courtroom looking on - including the musician Sting, who with his wife, Trudie Styler, has offered support for the villagers.

As part of his defense, Donziger has downplayed his role in leading the Ecuadorean litigation. In a witness statement filed with the court, Donziger emphasized that another lawyer in Ecuador, Pablo Ferjado, was the lead attorney and that Donziger served at the pleasure of the plaintiffs.

"I work for them; they do not work for me," Donziger testified.

During 90 minutes of questioning, Mastro sought to undermine that claim, focusing on the fees Donziger stands to collect if the judgment is enforced.

He asked Donziger about invoices that showed he was earning approximately $150,000 a year, while Ferjado was paid closer to $24,000.

"But you say you work for him," Mastro said. "He doesn't work for you?"

"Yes," Donziger said.

"He must be a very generous boss," Mastro said sarcastically, earning a rebuke from the judge.

Mastro also pointed to Donziger's 2011 retainer agreement, which called for him to receive nearly one-third of any contingency fee. Even with the award slashed in half last week, his portion would still be worth $600 million, Mastro said.

He quoted Donziger in various documents referring to himself as the "lead lawyer," saying he had an "integral role" in shaping strategy and giving instructions to the rest of the legal team.

Ferjado himself referred to Donziger as "cabeza," the Spanish word for head, and as "commander," though Donziger insisted the latter was a joke.