Convinced that terrorists are determined to disrupt the presidential election by attacking the United States, the government is mounting a massive homeland security effort as the run-up to the two presidential nominating conventions begins.

Unprecedented security arrangements have been made for the conventions in Boston and New York, where authorities believe al Qaeda may see an opportunity to disrupt and perhaps alter the outcome of the Nov. 2 election. But the planning extends to "symbolic events" such as Labor Day weekend, when large crowds will gather, and to Election Day.

"These events will bring to bear more protective measures than any in history, and while many of these measures can be seen, others like weapons of mass destruction detection equipment won't be seen," Homeland Security Department spokesman Brian Roehrkasse said of precautions for the conventions.

The effort stems from intelligence officials' conclusion that al Qaeda and like-minded groups, buoyed by the March 11 Madrid train bombings and the electoral defeat of Spain's government days later, are determined to launch something similar in this country. They base their conclusion on what they call incontrovertible classified intelligence that apparently includes electronic surveillance.

U.S. government officials say they have no intelligence specifying the time or place of an attack. Nor have they raised the color-coded threat index, or issued a dire warning such as the one Attorney General John D. Ashcroft gave before Memorial Day.

Federal agents will inspect sewers and weld shut manholes around the two convention centers, and remove mailboxes and most trash cans. Coast Guard craft will survey waterways for out-of-place boaters. Police officers will guard the ventilation systems of hotels where delegates stay, and dozens of highly trained emergency response teams will wait in warehouses in case of chemical or biological attack.

In Boston, some major highways, bridges and tunnels will be closed completely, including one interstate only feet from the FleetCenter, where the Democratic convention will be held July 26 to 29. That has prompted city officials to urge downtown workers to stay home that week. Riders of the city's subways will be subject to random searches of their bags.

New York, where Republicans will gather from Aug. 30 to Sept. 2, will be patrolled by more than 10,000 uniformed officers, and thousands of additional federal personnel on the streets, in the air and on the water.

At the same time, government officials fear that terrorists may avoid that security buildup and strike "soft" targets such as hotels or department stores -- across town or across the country -- as the conventions are underway.

Homeland security officials believe New York is a far more likely target than Boston because al Qaeda has consistently set its sights on that city, and because terrorists are galvanized by the prospect of striking the nation's top political leadership, from President Bush on down, U.S. officials said.

While Bostonians are preparing for grievous inconvenience during the Democratic convention, federal officials said they view the GOP convention as considerably more challenging for security personnel.

Immense crowds of anti-Bush protesters -- estimated by organizers at 250,000 to 1 million -- are expected in New York. City officials say the 10,000 officers on duty will be engaged in a delicate balancing act, simultaneously trying to guard against a terrorist attack and civil disobedience, while guaranteeing the First Amendment rights of demonstrators and trying to minimize disruption to New Yorkers.

Organizers of the largest protests express deep mistrust of the federal government and New York police for sounding what they believe are overblown alarms about terrorism to justify repressive security procedures and stifle dissent.

"There's an element of lies and deception," said Tanya Mayo, national organizer for Not in Our Name, a group organizing one of the biggest antiwar rallies.

The conventions have been designated National Special Security Events, which gives the U.S. Secret Service the lead in coordinating security. The Secret Service has long-standing expertise in heading off violence through intelligence-gathering and thorough planning, officials said.

Authorities are not simply hardening targets. The FBI and the Homeland Security Department recently established the "2004 threat task force." Armed with intelligence information, the FBI is trying to develop sources by interviewing people in the Muslim community and is reviewing previously collected intelligence to try to connect it to the current threat. The bureau also is making a major effort to locate seven suspected al Qaeda associates who it has said have the ability or intent to harm the United States.

U.S. intelligence officers said they noted a sharp increase in electronic "chatter" among al Qaeda followers about election-related attacks in the United States after the bombings of Spanish commuter trains killed 191 people and contributed to the fall of Spain's government.

U.S. officials say the current intelligence warnings in some respects resemble threats picked up in December, which led to the Dec. 21 imposition of an orange, or "high risk," terror alert that lasted three weeks. The intelligence, which officials said was gathered from several independent sources, indicated that al Qaeda sought to launch a terrorist strike that would cause at least 100,000 U.S. casualties, officials said.

That led officials to ratchet up security at nuclear and chemical plants, and to dispatch teams with special equipment in search of nuclear weapons or radioactive material in several cities.

At this summer's Boston and New York gatherings, federal officials say they will so "harden" the FleetCenter and Madison Square Garden convention sites -- by extending security perimeters for blocks around both buildings, strictly limiting access to credentialed people, and other measures -- that they doubt an attack could be mounted on the buildings.

Boston and New York will host the largest collection of bomb-sniffing dogs in history, officials say. Coast Guard boats will ply their waters, including the Charles River, which runs only blocks from the FleetCenter. Other teams will check delegate hotels for explosives, prompted by an attack in May that killed the Chechen president and 13 others with bombs embedded earlier in a stadium reviewing stand.

Guards will be added at New Jersey chemical plants upwind of Manhattan and other facilities near both cities. Both regions will deploy medical surveillance systems that require clinics to report sore throats and pharmacies to report drug sales -- both possible early warnings of biological attacks.

Delegate hotels, the sites for many of the 2,000 parties scheduled during those weeks, are, for security purposes, almost extensions of the convention sites themselves. Jimmy Chin, who chairs the New York Hotel Association's safety panel, said hotel security teams will work closely with law enforcement to spot protesters who might try to sneak into lobbies or book rooms.

The security plans for the two conventions differ in some ways. Although New York plans to close a few streets around midtown Manhattan during the late afternoons and evenings, Boston will shut down 40 miles of roadways. One reason is the need to close Interstate 93, whose guardrails are only 10 feet from the FleetCenter's walls. That led in part to decisions to close or place restrictions on other major regional roadways, the Tobin Bridge, the Sumner Tunnel from Logan International Airport and a small stretch of Interstate 90, the Massachusetts Turnpike.

Boston officials say they must reduce traffic by half to avoid gridlock and are urging companies to allow employees to work from home. Hospitals are being urged to postpone non-emergency surgery, and tenants have been asked to delay apartment moves. Mayors of surrounding towns such as Somerville have threatened to bar overflow traffic if it ties up their streets.

"Yes, traffic will be slower than usual," Boston Mayor Thomas M. Menino said, "but I predict this city will work.

The Secret Service never proposed closing New York's Penn Station, which sits directly below Madison Square Garden, where the Republican convention will be held. It is the nation's busiest rail hub, with 425,000 passengers a day moving through on subway and rail lines to Long Island, New Jersey and elsewhere.

Instead, New York is using the sheer size of its police force -- 36,000 uniformed officers as opposed to Boston's 2,200 -- to keep Penn Station open. Along with hundreds of New York, New Jersey and Connecticut state troopers, thousands of NYPD officers, many with trained sniffing dogs, will ride the rails day and night during the convention.

"Our greatest asset is our size, which gives us tremendous flexibility and visibility," said Paul Browne, NYPD's deputy commissioner of public information.

With hundreds of bridges, tunnels, tourist attractions and critical infrastructure sites to protect, the NYPD is canceling days off during the convention. It doesn't help that the U.S. Open tennis tournament in Queens overlaps with the GOP gathering.

But New York Mayor Michael R. Bloomberg has repeatedly said that the convention will register barely a blip in the city. "If you don't live or work in the Garment District, you won't even know there's a convention in town," he has said.

U.S. officials said they fear terrorists will somehow make use of the rallies -- either by hiding their activities while surrounded by protesters, or blowing up tightly packed crowds of demonstrators. Some antiwar groups have announced plans for "direct action" protests that include trashing businesses such as Starbucks and the Gap.

Despite complaints by the New York Civil Liberties Union, authorities said they likely will once again set up barricades to separate protesters from outsiders and search knapsacks and other bags carried by demonstrators.

Staff writer Susan Schmidt contributed to this report.

A transit authority officer patrols with his dog through New York's Penn Station beneath Madison Square Garden.