MANILA, Philippines — At a quarter to 8 on a recent morning, Simeon Marcelo leaned back in his silver van. One escort van drove ahead of his vehicle, another behind; the whole entourage included eight plainclothes bodyguards. A bulletproof vest sat unused next to Marcelo. It was, he said, too cumbersome to wear.

"If it's your time to go, it's your time," the country's top graft buster said.

A mild-mannered former seminarian and corporate lawyer who is now the Philippines' ombudsman, Marcelo is taking on the most powerful vested and entrenched interests in a country that perennially ranks among the world's most corrupt.

This is the place where two decades ago longtime dictator Ferdinand Marcos amassed a fortune in public money while his wife, Imelda, famously filled her closet with thousands of pairs of shoes.

Quietly, methodically, Marcelo is targeting the most corrupt agencies, daring to prosecute even members of the historically untouchable military.

Marcelo is confronting a highly skeptical public and an officialdom steeped in the culture of bribes, gifts and extortion. He has one field investigator for every 17,000 public employees. And he gets weekly death threats, some of which are ominously precise about his whereabouts.

"He faces an enormous battle, probably a lot more than what we were facing in Hong Kong in the 1970s," said Tony Kwok, the former deputy commissioner of the Hong Kong Independent Commission Against Corruption, who helped curb official dishonesty in the territory and is advising Marcelo.

Without greater political will in the Legislature and judicial reform, Kwok said, Marcelo's efforts will be for naught.

But analysts say Marcelo is making a difference, hiring young, idealistic lawyers and banning the favor seekers who once milled around the office of the ombudsman. Since taking office in October 2002, he has doubled the prosecutorial staff, more than doubled the investigative staff and raised the conviction rate from 15 percent in 2001 to 42 percent in the first quarter of 2005.

"He has restored credibility to that organization," said Marites Vitug, editor in chief of Newsbreak magazine here.

Until Marcelo took office, no one had ever prosecuted a high-ranking active-duty military official for graft. Now, the ombudsman is investigating at least 10 senior officers.

At 51, Marcelo has ulcers and high blood pressure, not helped by an 80-hour workweek. He has put away the Ferragamo neckties he used to wear as a corporate lawyer and favors simple white shirts befitting a public servant on an $8,400 annual salary. He jokes that his wife, a vice president at a cement-manufacturing firm, supports him.

He doubts that he'll serve out his seven-year term as ombudsman. But before he leaves, he said, he wants to win the big one: the case against former President Joseph Estrada, ousted in January 2001 in the midst of impeachment proceedings for alleged corruption.

Marcelo is arguing that Estrada has amassed $74 million in illegal gambling kickbacks and tobacco-tax diversions. He knows the case well. A volunteer prosecutor during the House impeachment hearings, he examined a key prosecution witness who charged that Estrada got $12.5 million in gambling protection money and tax skimming. Before the Senate trial was completed, massive protests and the withdrawal of military support forced Estrada out.

Estrada, 68, maintains his innocence.

Marcelo began his anti-corruption drive in 2001 when, as solicitor general, he revived a long-dormant case against Marcos, who had been forced out in 1986, and his wife.

Resisting unofficial offers from the Marcos camp to settle the case, he forced the return of $680 million that the couple had stashed in a Swiss bank account. It was the largest repatriation of ill-gotten wealth in history.

When Marcelo first took office, skeptics questioned his independence. He hailed from the law office of Carpio, Villaraza and Cruz, known simply as "the firm" because of its links to President Gloria Macapagal Arroyo, who succeeded Estrada. The firm had served as the Arroyo family's private counsel before Arroyo became president.

Since taking office, Marcelo has shunned social affairs where he might run into his legal buddies or administration officials.

He has asked his former firm not to handle cases tried by his agency. In 2003, he filed corruption charges against 26 Arroyo appointees accused of grossly inflating the cost of a highway, and he is investigating charges of illegal gambling against Arroyo's son, a congressman.

"People trust him a lot more now than they did when he first came in," said Guillermo Luz, executive director of the influential Makati Business Club.