WASHINGTON — Tim Russert, who pointedly but politely questioned hundreds of the powerful and influential as moderator of NBC’s “Meet the Press,” died today of an apparent heart attack. The network’s Washington bureau chief was 58.

In addition to his weekly program, Russert made periodic appearances on the network’s other news shows, was moderator for numerous political debates and wrote two best-selling books.

President Bush, informed while at dinner in Paris, swiftly issued a statement of condolence that praised the NBC newsman as “an institution in both news and politics for more than two decades. Tim was a tough and hardworking newsman. He was always well-informed and thorough in his interviews. And he was as gregarious off the set as he was prepared on it.”

NBC interrupted its regular programming and in the ensuing moments, familiar faces such as Tom Brokaw, Andrea Mitchell, and Brian Williams took turns mourning his loss.

Mr. Williams called him “aggressively unfancy.”

“Our hearts are broken,” Ms. Mitchell said.

Russert, of Buffalo, N.Y., took the helm of the Sunday news show in December 1991 and turned it into the nation’s most widely watched program of its type. His signature trait there was an unrelenting style of questioning that made some politicians reluctant to appear, yet confident that they could claim extra credibility if they survived his grilling intact.

He was also a senior vice president at NBC, and this year, Time Magazine named him one of the 100 most influential people in the world.

Russert had Buffalo’s blue collar roots, a Jesuit education, a law degree, and a Democratic pedigree that came from his turn as an aide to the late Senator Moynihan of New York.

One of his books, “Big Russ and Me,” was about his relationship with his father.

He was married to Maureen Orth, a writer for Vanity Fair Magazine. The couple had one son, Luke.

The president of the Gridiron Club, an organization of journalists, Carl P. Leubsdorf, said in a statement, “It was a measure of the degree to which Tim Russert was respected in the journalistic world that he was the first broadcaster elected to membership in the Gridiron Club after the rules were changed in 2004 to end our century-old restriction to print journalists.”

“He was an enthusiastic member and a willing participant in our shows. His fellow Gridiron members join with all of those who knew and respected Tim in mourning his untimely death.”

“Tim will be sorely missed because his years as Senate staffer and probing TV journalist gave him special insights on political and governmental issues,”Senator Specter, a Republican of Pennsylvania, said. “Had he chosen law as a career, his cross-examination would have made him a star in that field as well.”

“It is my sad duty to report this afternoon” that Russert collapsed and died while working in the network’s Washington studios, Mr. Brokaw said when he came on the air.

The network said on its Web Site that Russert had been recording voiceovers for this Sunday’s “Meet The Press” when he was stricken.