New York Mayor Michael Bloomberg signed an order for city agencies "to take whatever steps necessary to protect the wellbeing and safety of city residents in the face of excessive heat" as the city baked under the sun.
Temperatures were expected to climb to around 38 centigrade in New York this week while the heat index, which takes moisture readings into account, could reach 43 degrees centigrade, the US weather service said.
The mayor's office urged city residents to conserve on their electricity use to avoid the power outages that paralysed a part of the borough of Queens last month for more than a week.
Bloomberg's office said any individual who "knowingly" violates the request to conserve energy - for example by running their air-conditioner while no one is at home - could be charged with a misdemeanour.
The city said it was also setting up 383 cooling centres in seniors centres and community facilities across New York's five boroughs, and ordering its public pools to stay open into the late evening so that New Yorkers could cool off after work.
The temperature in Washington, DC was forecast to top off at 38 centigrade and reach similar heights tomorrow, as city authorities used television announcements to urge people with respiratory problems not to venture outdoors.
In the midwestern city of Chicago, where about 800 died in a 1995 heatwave, several hundred elderly people were evacuated from their buildings when an underground distribution line failed, depriving 20,000 people of electricity and air-conditioning.
In California, where temperatures have cooled since a historic 15 day heatwave, 136 people died from the high temperatures, according to preliminary statistics, many of them elderly.
At least three immigrants were also found dead from the heat after having crossed the border illegally from Mexico.