The mercury rose above 40C for the first time on record in Britain, recorded at Heathrow in London, with red and amber weather warnings in place across the country.
Temperatures were cooler in Sunderland, though still rose to around 32C.
Activities were cancelled and various measures were brought in by schools and employers to keep people safe.
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Amid fears the heatwave could lead to hundreds of deaths and hospital cases, health teams have been calling vulnerable people with pre-existing medical conditions to offer advice.
Rail operators urged passengers not to travel unless necessary.
Metro services were suspended on stretches of the network due to overhead wires sagging in the heat in multiple locations.
The East Coast Main Line closed from 12pm to 8pm for all locations between London King’s Cross and York and Leeds.
Fears over railway tracks buckling let to transport chiefs enforcing the speed restrictions.
Northern rail, which runs services connecting Tyneside and Wearside stations with the rest of the North, warned passengers blanket speed reductions and a limited timetable were in place.
Kerry Peters, regional director for Northern, said: “We don’t issue ‘Do Not Travel’ notices lightly, but given the guidance published by Government and expected disruption caused by extreme temperatures, we feel this is the safest advice for our customers."
In some parts of the country, there were reports of people drowning as they tried to escape the heat.
Sunderland City Council’s director of public health, Gerry Taylor, issued advice on staying safe ahead of the heatwave, adding that some people were more vulnerable than others to extreme temperatures.
"This includes babies and young children, older people and those with underlying health conditions,” she said.
"While many of us are enjoying the warmer weather, it's really important that we do everything we can to stay safe in the sun and to look out for those who may be more vulnerable or struggle to stay cool and hydrated in hot weather.
"So if you know older friends, family or neighbours who fall into these groups it may be helpful to keep an eye on them.”
Temperatures for the rest of the week are set to be cooler, forecast to be no 20C in Sunderland on Wednesday and 17C on Thursday.