Fast-moving thunderstorms pummeled southeastern Texas on Thursday for the second time this month, killing at least four people, blowing out windows in high-rise buildings, downing trees and knocking out power to more than 900,000 homes and businesses in the Houston area.

Officials urged residents to keep off roads, as many were impassable and traffic lights were expected to be out for much of the night.

"Stay at home tonight. Do not go to work tomorrow, unless you're an essential worker. Stay home, take care of your children," Houston Mayor John Whitmire said in an evening briefing. "Our first responders will be working around the clock."

The mayor said four people died during the severe weather. At least two of the deaths were caused by falling trees, and another happened when a crane blew over in strong winds, officials said.

Streets were flooded, and trees and power lines were down across the region. Whitmire said wind speeds reached 160 kph, "with some twisters." He said the powerful gusts were reminiscent of 2008's Hurricane Ike, which pounded the city.

Hundreds of windows were shattered at downtown hotels and office buildings, with glass littering the streets below, and the state was sending Department of Public Safety officers to secure the area.

"Downtown is a mess," Whitmire said.

There was a backlog of 911 calls that first responders were working through, he added.

At Minute Maid Park, home of the Houston Astros, the retractable roof was closed due to the storm. But the wind was so powerful it still blew rain into the stadium. Puddles formed on the outfield warning track, but the game against the Oakland Athletics still was played.

The Houston Independent School District canceled classes Friday for some 400,000 students at all its 274 campuses.

The storm system moved through swiftly, but flood watches and warnings remained for Houston and areas to the east. The ferocious storms moved into neighboring Louisiana and left more than 215,000 customers without power.

Flights were briefly grounded at Houston's two major airports. Sustained winds topping 96 kph were recorded at Bush Intercontinental Airport.

About 900,000 customers were without electricity in and around Harris County, which contains Houston, according to poweroutage.us. The county is home to more than 4.7 million people.

The problems extended to the city's suburbs, with emergency officials in neighboring Montgomery County describing the damage to transmission lines as "catastrophic" and warning that power could be impacted for several days.

Heavy storms slammed the region during the first week of May, leading to numerous high-water rescues, including some from the rooftops of flooded homes.