By Ian Simpson

WASHINGTON (Reuters) - Flooding from torrential rain killed two people in Ellicott City, Maryland, with floodwaters that tore through the U.S. town's historic downtown collapsing a street and sweeping away cars.

Ellicott City was pounded by almost 6 inches (15 cm) of rain in two hours late on Saturday, sending the Tiber, a tributary of the Patapsco River, out of its banks, officials said.

Howard County Executive Allan Kittleman said the flooding in Ellicott City, about 35 miles (56 km) northeast of Washington, was worse than that from Hurricane Agnes in 1972.

"I don't believe there's ever been a flood and the devastation that we've had overnight in Ellicott City," he said in an interview with Baltimore's WBAL NewsRadio.

County spokesman Andy Barth said a man and a woman were killed. The woman's body was recovered from the river overnight.

Barth said every business near the river on the town's historic Main Street had suffered major damage, including building fronts torn off and doors stripped away.

Governor Larry Hogan declared a state of emergency, as did Kittleman. The declarations allow aid to be released more quickly for Ellicott City, which has a population of about 65,000.

Firefighters rescued about 120 people and emergency workers were also dealing with a water main break, Howard County said in a statement. On a video posted online, men formed a human chain to get a woman trapped by raging waters out of her car.

Television footage showed a downtown street collapsed, power poles down, mud-covered roads and cars tossed on one another. The heavy rain was part of a system of thunderstorms that moved through the region.

(Reporting by Ian Simpson in Washington and Alex Dobuzinskis in Los Angeles; Editing by Adrian Croft and Marguerita Choy)