A stadium worker exits an empty Wrigley Field during Monday's tornado warning. (M. Spencer Green/Associated Press)

A powerful storm halted Monday night's game between the Houston Astros and Chicago Cubs, and fans at Wrigley Field were advised to take cover on the concourse during the early stages of the delay.

After the rain subsided and the field was made playable by the grounds crew, the game resumed after a delay of two hours, 45 minutes.

But the teams played only 50 minutes before crashing thunder and lightning caused another halt in play.

When a thunderous boom roared with one out in the eighth inning, Astros first baseman Lance Berkman headed for the dugout and the umpires waved the rest of the Astros off as well as the rain began to pour again.

Houston led the game 2-0 in the bottom of the eighth, and won the game by that score when the game was finally called after a 39 minute delay.

The game was initially stopped at 7:39 p.m. local time because of rain, with Houston leading 2-0 in the top of the sixth.

The National Weather Service issued a tornado warning for downtown Chicago and a siren went off in the neighborhood surrounding Wrigley Field at about 8:05 p.m.

There was an announcement over the ballpark's public address system and a message on the large scoreboard in centre field, advising fans to seek shelter on the concourse.

Most left the upper stands and went into the lower deck, though some stayed in their seats.

A couple of fans ran on the field and started body sliding on the tarp.

There were oohs and aahs from the crowd as thunder and spectacular lightning crackled in the sky.

The siren ceased about 8:10 p.m., then resumed and stopped again.

After the warnings, rain came in a torrent across the field with high winds, reducing visibility to zero.

The warning track and the area behind home plate were soaked within minutes, as was the front row of the press box.

"I have never seen anything like it," said Bob Sejnoha, a fan at the game.

"This is unprecedented. I don't think it has ever happened at Wrigley."

In preparation as the storm approached, flags commemorating former Cubs greats were taken down off the foul poles at Wrigley Field.

About 90 minutes after the game was halted, the wind and rain subsided considerably, though lightning and thunder continued.

Fans were allowed to return to their seats in the bleachers and grandstands.

The grounds crew swept up debris from behind home plate and worked to clear standing water from the warning track.

The National Weather Service did not immediately confirm any tornado touchdowns.

But it said trained spotters reported high-rotation winds in DeKalb and Kane counties in northeastern Illinois.

Meteorologist Brad Churchill said 96 km/h to 128 km/h winds were expected.

There were no immediate reports of injuries, Chicago Fire Department officials said.

Passengers at O'Hare International Airport were evacuated into the lower levels of the complex's buildings.

Travellers who had already boarded airplanes were taken off as the storm struck, then led to the lower levels as a precaution.

The storms did not cause any damage or injuries at O'Hare or Midway Airport, said Department of Aviation spokesman Gregg Cunningham.

More than 100,000 people in northern Illinois were without power, said ComEd spokesman Joe Trost.