Military aircraft drowned out the sound of surf, gunboats cruised a historic riverfront and Secret Service agents guarded a beach road as security tightened with President Bush's arrival for this week's summit of world leaders.

Concrete barriers, metal fencing and checkpoints were put into place around key buildings and routes in final preparations for the Group of Eight (search) Summit starting Tuesday on adjoining Sea Island.

Thousands of police officers and National Guard troops patrolled roadways and bridges.

U.S. officials have said the summit could be one of several high-profile U.S. events targeted this summer by Al Qaeda terrorists.

Anti-globalization protests, on the other hand, are likely to be mostly low-key.

"If there was going to be a terrorist, I'm sure they would have come here long before all the security arrived last week," said Kathy Tharpe, who operates a fruit-and-vegetable stand next to the St. Simons Island airport.

While some island residents have left for the week, Tharpe plans to keep her business open unless her daily deliveries of produce are no longer allowed onto the island.

In 1999, anti-globalization demonstrators set fires in the streets of Seattle, and at the 2001 G-8 summit in Genoa, Italy, police fatally shot one demonstrator and arrested 300 others.

Several protest groups planned to meet Monday at a seaside park on St. Simons and a community college in Brunswick. Both gatherings were being billed as nonviolent festivals, featuring music and speakers. Protesters in both cities planned to hand out pamphlets to local residents and police.

Police have warned that traffic will be snarled by military checkpoints throughout the area. Only those with high-level credentials were being allowed onto Sea Island, the secluded resort where the leaders will gather Tuesday through Thursday, and where President Bush arrived Sunday evening from his D-Day trip to Europe.

Somewhere between 10,000 and 20,000 federal, state and local officers were on duty in Sea Island, adjoining St. Simons Island, nearby Brunswick on the mainland and 80 miles north in Savannah, where thousands of journalists were staying.

In Savannah, National Guard troops in sand-colored Humvees began cruising the cobblestone streets and oak-shaded squares of the historic downtown while helicopters hovered overhead.

Coast Guard boats with mounted machine guns patrolled the Savannah River between the summit's media center on Hutchinson Island and the city's riverfront promenade of oyster bars and T-shirt shops.

"It's a little off-putting for people visiting," said Liz Demos, owner of a downtown home-furnishing store. "It's hard to enjoy the city when there's Humvees passing you."

The city's $1 billion tourism industry was taking a hit as businesses geared toward tourists saw a dramatic dip in sales.

On St. Simons, the thundering of military aircraft drowned out the sounds of the ocean.

On the four-mile causeway linking Brunswick to St. Simons, the military started searching vehicles Saturday.

The security was even more stringent along the road from St. Simons to Sea Island, the resort where the leaders will gather. A long, parallel set of 7-foot-high, metal-mesh fences protected the only road that leads to the island, and black-clad Secret Service agents stood guard.

The surrounding waters are under close watch as well. Twenty-five-foot Coast Guard patrol boats armed with M240 machine guns are on patrol, with their crews armed with M-16 assault rifles and shotguns.

"Some of these security measures are visible," said Secret Service spokesman Thomas Mazur. "However, many of them are not."