By ELISABETH BUMILLER and LARRY ROHTER

MAR DEL PLATA, Argentina, Nov. 4 - President Bush's foreign and domestic troubles trailed him to the opening day of an international summit here as tens of thousands protested in the streets and Mr. Bush deflected questions about his chief political aide, Karl Rove, who remains under investigation in the C.I.A. leak inquiry.

At a brief news conference with American reporters today at the Sheraton Mar Del Plata, Mr. Bush was asked four times about Mr. Rove, and four times refused to answer. The president did not take the opportunity to offer a public endorsement of Mr. Rove, nor did he address speculation in Washington about whether Mr. Rove would stay as his deputy chief of staff.

Asked if there were discussions at the White House about whether or not Mr. Rove would remain in his job, Mr. Bush replied that "the investigation on Karl, as you know, is not complete, and therefore I will not comment about him and/or the investigation."

Mr. Bush calmly added, "I understand the anxiety and angst by the press corps to talk about this." But he called the C.I.A. leak inquiry "a very serious investigation," and said that the White House is "cooperating to the extent that the special prosecutor wants us to cooperate."

At the same time, Venezuela's populist president, Hugo Chávez, rallied some 25,000 protesters in this beach resort's main soccer stadium. He declared a free trade accord backed by Mr. Bush as dead and accused the Pentagon of having a secret plan to invade his oil-rich country.

"If it occurs to U.S. imperialism, in its desperation, to invade Venezuela, a 100-years' war will begin," Mr. Chávez declared to cheers.

A few blocks from the hotel where the summit conference was taking place, some protesters threw stones and set fires this afternoon, and the police fired tear gas canisters to break up the demonstrations. Television images showed riot police arriving in vans and on motorcycles and horseback and massing near the crowds.

Several hundred demonstrators wearing bandanas or masks over their faces brandished clubs or fired slingshots at the police and set fire to a bank, according to reports by news agencies. The rioting broke out after a much larger, peaceful protest march involving several thousand people, the agencies said.

President Bush arrived here on Thursday night after one of the worst weeks of his presidency, only to be greeted by strong anti-American sentiment and taunts from Mr. Chávez.

Today, Mr. Bush said that he and Argentina's president, Néstor Kirchner, had agreed in talks that the United States' role in the region could be constructive and positive. Mr. Bush stressed the need for wise decisions to attract investments.

Standing next to President Kirchner, he also made what appeared to be a reference to the protests.

"It's not easy to host all these countries," he said, addressing Mr. Kirchner. "It's particularly not easy to host, perhaps, me," he said, drawing laughter.

The Summit of the Americas, a two-day, 34-nation gathering, opened to officially focus on creating jobs and promoting democracy.

Mr. Chávez, who has repeatedly accused the Bush administration of trying to assassinate him and invade his oil-producing country, is using the international summit meeting here to protest the administration's free trade message and to attempt a showdown with Mr. Bush, the man the Venezuelan government calls "Mr. Danger."

He said this week that his main goal at the meeting was the "final burial" of the proposed Free Trade Area of the Americas accord, which is already stalled.

"I think we came here to bury F.T.A.A.," Mr. Chávez said today, according to remarks reported by Reuters. "I brought my shovel."

Mr. Bush said today he would be "polite" when he meets Mr. Chávez.

The White House strategy is to ignore Mr. Chávez as much as possible.

"President Chávez has been pretty vocal about how he sees the summit and what he hopes to achieve at the summit," Thomas A. Shannon, the assistant secretary of state for inter-American affairs, told reporters on Air Force One on Thursday as it headed for Argentina. "I mean, he's going to behave the way he wants to behave."

Earlier this week, Mr. Bush did not denounce a longstanding request from Mr. Chávez that the Argentine government build a nuclear reactor in Venezuela for energy production.

"I guess if I were a taxpayer in Venezuela, I would wonder about the energy supply that Venezuela has," Mr. Bush said in an interview at the White House on Tuesday with a group of reporters from Latin American publications. "But maybe it makes sense." Mr. Bush added that "it's the first I've heard of it."

A little more than 24 hours later, Stephen J. Hadley, the national security adviser, appeared to backtrack when he noted that Mr. Chávez had asked a number of countries to build a nuclear reactor in Venezuela, and that he was far from a deal.

"I think that's because people recognize that it would be problematic for Chávez to be in the nuclear business, if you will," Mr. Hadley said, adding that "this trip, this summit, is not about Hugo Chávez."

But behind the scenes on Thursday, the United States and Venezuela were intensely jostling for advantage. As a result, negotiators were still struggling to come to an agreement over the final text of a joint communiqué, meant to be based on a consensus, that the leaders here hope to issue when the meeting ends on Saturday.

In a section on job creation, United States representatives have suggested taking note of "the 96 million people who live in extreme poverty" in Latin America and the Caribbean, subsisting on $1 a day or less. But Venezuela would agree to that statement, Latin American diplomats said, only if the following phrase were also included: "while in the United States there are 37 million poor."

The deepest disagreements had to do with the issue of free trade, which Mr. Bush has offered as the key to economic growth in the hemisphere. Washington is said to be pushing to issue a statement favoring the resumption of negotiations aimed at establishing the free trade accord, abbreviated F.T.A.A., but has met resistance not only from Venezuela but from Brazil and Argentina, too.

"The only language which is a problem is F.T.A.A.," said José Miguel Insulza, the secretary general of the Organization of American States. "We're moving toward a solution, not in the meeting but in the corridors."

At a parallel "People's Summit" in Mar del Plata on Thursday, organized by a coalition of left-wing, indigenous and antiglobalization groups, American proposals on free trade also came in for criticism, as did Mr. Bush himself.

"We Said No and No Means No: No to Bush, No to F.T.A.A. and No to Repaying the Debt," read one large banner at the conference, held in a group of tents and classrooms on the campus of a local university. Several thousand people attended.

"We've had enough of neo-liberalism and the damage it has inflicted on our societies," said Juan Montenegro, who came from Buenos Aires to take part. "Bush is trying to destroy Iraq with bombs and guns and Latin America with an economic program that will rob us of our sovereignty."

The "antisummit" began early in the week and was expected to culminate today in mass protest marches, led by Adolfo Pérez Esquivel, the Nobel Peace Prize winner, and Diego Maradona, the soccer idol.

Christine Hauser contributed reporting for this article from New York