By Gabriella Borter

TITUSVILLE, Fla, Sept 1 (Reuters) - Hurricane Dorian was set to slam the Bahamas on Sunday as a dangerous category 4 storm, pounding the islands with up to two days of torrential rain, high waves and damaging winds before taking aim at the U.S. mainland.

Bahamas Prime Minister Hubert Minnis begged residents of Abaco and Grand Bahamas to head for the main island to escape the "devastating, dangerous" storm.

"I want you to remember: homes, houses, structures can be replaced. Lives cannot be replaced," he told a news conference on Saturday, adding that 73,000 people and 21,000 homes were at risk to storm surges of up to 15 feet (4.6 meters).

Late on Saturday, the Miami-based National Hurricane Center (NHC) said Dorian was packing maximum sustained winds of 150 mph (240 kph) and was about 155 miles (245 km) east of Abaco.

After churning over the Bahamas, it is expected to veer northwest and could spare Florida a direct hit. Communities further north in Georgia and South Carolina raised alert levels on Saturday, with residents filling sandbags as authorities tested infrastructure and hurricane drills.

Despite the more favorable forecasts for Florida, town authorities there told people to remain vigilant and a tropical storm watch was issued for a more than 120-mile stretch (193 km) of the state's coast, meaning sustained winds of up to 73 mph (117 kph) were possible within 48 hours.

Most tourists who planned to leave the Bahamas got out before the main airport closed on Friday night.

Jeffrey Simmons, the deputy director of Bahamas' department of meteorology, said Dorian will cause prolonged periods of large swells and storm surges along the north coast of Grand Bahama and the north and east coast of Abaco.

"We are asking residents in those areas to leave the coastline," Simmons added. "We expect a storm surge of up to 15 feet. In addition to that we have a spring tide that can increase the surge by 2 to 3 feet."

Potential damage to the Bahamas from Dorian could be exacerbated by the fact that its westward motion is forecast to slow, keeping it over the islands for longer, the Bahamas National Emergency Management Agency said in a statement.

Grand Bahama and Abaco are hubs of the Bahamas' thriving tourism industry. But after days of evacuations only 26 tourists are left on Grand Bahama, authorities said on Saturday.

"We have been for the last few days asking all tourists to leave the island in anticipation of the hurricane," said Kwasi Thompson, minister of state for Grand Bahama. (Reporting by Gabriella Borter in Titusville; Additional reporting by Zachary Fagenson in Jacksonsville and Anthony Esposito in Mexico City; Writing by Daniel Wallis; Editing by Clarence Fernandez)