Houston is under a boil water notice after a power outage caused low water pressure across the city, according to Houston Public Works.

Power went out at a water purification plant at about 10:30 a.m Sunday. Power and water pressures have since been restored, but the boil water notice is still in effect.

"We believe the water is safe but based on regulatory requirements when pressure drops below 20 psi we are obligated to issue a boil water notice," Houston Mayor Sylvester Turner said on Twitter.

The city has to wait at least 24 hours after the Texas Commission on Environmental Quality has tested water samples and deemed it safe to drink to lift the notice, Turner said.

Houston Water and Wastewater Utility serves 2.2 million customers a day.

The Houston Independent School District has canceled classes for Monday. All schools, offices and facilities will be closed.

During a boil water notice, impacted residents should bring water used for cooking, drinking or hygiene to a vigorous boil for at least two minutes and allow it to cool before using it to properly kill harmful bacteria.

Water supplied for ice makers and water dispensers on refrigerators should not be used during a boil water notice.

Residents can alternatively use bottled water for their needs.