HAVERHILL — A late-afternoon thunderstorm took its toll on Haverhill yesterday, knocking down trees and disrupting electrical power for several hours for up to 1,700 customers.

As of 9 last night, 600 homes were still without power, many of them in the Ward Hill area.

"Haverhill was the only place in the area where we had a lot of power outages," said David Graves, a spokesman for National Grid.

The storm cut off power for one home in Amesbury and 21 customers in West Newbury, he said.

"We hope to have everybody up tonight sometime," Graves said.

The National Weather Service issued a severe thunderstorm warning at 4:10 p.m. that the Haverhill area would be in for nickel-size hail and wind gusts of more than 60 mph by 4:30 p.m. The storm was forecast to move to West Newbury and Amesbury 10 minutes later.

"We don't have any reports about hail or the wind speeds," said meteorologist Bill Simpson at the National Weather Service bureau in Taunton.

"But trees down and power outages are a reflection of the wind damage," Simpson said.

Not long after the Weather Service broadcast the thunderstorm warning, Haverhill police were forced to reroute traffic on busy Route 97 when a large willow tree — about 50 feet tall and about 5 feet around — fell across the road at the intersection of Lake Street and Route 97 (Broadway).

Shortly before 5 p.m., police had blocked off Route 97 and were rerouting traffic onto Lake Street. Police said an early report that the tree had fallen onto a car, trapping people inside the vehicle, was false. The car's driver had tried to go around the tree but got stuck in mud at the side of the road.

The tree snapped electrical wires off utility poles when it fell, causing a power failure in the area. Willow trees have shallow roots and are known to be knocked over by strong winds relatively easily.

Dan Bird, owner of Broadway Deli Variety, 864 Broadway, which is at the intersection, said the store's lights went out and someone said to him, "A tree went down."

Westbound traffic on Lincoln Avenue heading into the city was detoured onto side streets around a downed power line in front of St. John the Baptist Church, at 110 Lincoln Ave., yesterday afternoon. Eastbound traffic was not affected by the downed line.

The dark sky, rain and lightning caused further concerns in Haverhill when there was a report of lightning hitting a house at 890 Hilldale Ave. at 5:08 p.m. Police and firefighters were investigating but there appeared to be no damage to the building.

At 5:45 p.m., there was a report of a tree across a power line at 90 Kingsbury Ave.

In low-lying areas like those on Primrose Street, there was heavy flooding. In one location, a car had to be pulled out of the deep water.

The thunderstorm started in the southwestern part of New Hampshire, swept through Haverhill and then moved on to Manchester-by-the-Sea. Southern New Hampshire communities were spared the kind of storm damage that took its toll on Haverhill.

In Derry, N.H., a tree fell partially across Route 28 near Promises To Keep early last night, blocking the entire lane of travel for traffic headed to Windham. But police reported the downed tree was cleared away quickly and said they weren't aware of any power outages or other problems.

Closer to Boston the strong thunderstorms were blamed for power outages and street flooding, and may have ignited at least two building fires as well.

NStar spokeswoman Caroline Allen said about 25,000 customers lost power at the height of the storms, with most of the outages concentrated in Newton, Waltham and Watertown.

Allen said most of the outages were caused by lightning strikes or tree limbs coming down on power lines.

The Boston Fire Department said lightning may have started a two-alarm fire in Back Bay that sent four firefighters to the hospital with minor injuries. Lightning also was the likely cause of a smoky blaze in a multifamily home in Cambridge.

The Coast Guard and other agencies rushed to the aid of eight sailboats in Boston Harbor that were caught in a storm. A Coast Guard spokeswoman said none of the 57 people aboard the boats was injured.

The weather service forecasts a chance of thunderstorms and showers today through Monday.

Material from the Associated Press is included in this report.