Concerned residents of the Toledo area descended on city stores, buying carts full of bottled water, bags of ice and flavored water and emptying shelves within hours of the advisory, which was issued overnight.
"It looked like Black Friday," said Aundrea Simmons, who stood in a line of about 50 people at a pharmacy before buying four cases of water. "I have children and elderly parents. They take their medication with water."
The city advised residents not to brush their teeth with or boil the water because that would only increase the toxin's concentration. Showers and baths are fine, the mayor said.
Toledo issued the warning just after midnight after tests at one treatment plant showed two sample readings for microsystin above the standard for consumption.
Mayor D. Michael Collins said at a news conference that city officials hope to know later Saturday how long the warning will stay in place. He asked residents not to panic.
Police officers were called to stores as residents lined up for water. "People were hoarding it. It's ridiculous," said Monica Morales, who bought several cases of bottled water before the store sold out of water a half-hour after opening.
Some neighboring communities that aren't connected to Toledo's water system were offering their water to people who brought their own bottles and containers.
A sample of water was being flown to the U.S. Environmental Protection Agency in Cincinnati for additional testing, officials said.
The city's advisory said Lake Erie may have been affected by a bloom of harmful algae that produces the toxin. Consuming the tainted water could result in vomiting, diarrhea and other problems.
The advisory covers city residents and those in Lucas County served by the city's water supply. The city said more tests are being run.
Many restaurants were closed because of the water warning and Toledo's public school system canceled all its events Saturday.
The University of Toledo closed its campus for the day and encouraged students who are from outside the Toledo area to return to their homes.
Operators of water plants all along Lake Erie, which supplies drinking water for 11 million people, have been concerned over the last few years about toxins fouling their supplies.
Almost a year ago, one township just east of Toledo told its 2,000 residents not to drink or use the water coming from their taps after tests on drinking water showed the amount of toxins had increased.
That was believed to be the first time a city has banned residents from using the water because of toxins from algae in the lake.
Most water treatment plants along the western Lake Erie shoreline treat their water to combat the algae. The city of Toledo spent about $4 million last year on chemicals to treat its water and combat the toxins.
The annual algae blooms have been concentrated around the western end of Lake Erie. The algae growth is fed by phosphorous mainly from farm fertilizer runoff and sewage treatment plants, leaving behind toxins that can kill animals and sicken humans.