The temperature exceeded the 30-degree mark in Tohmajärvi, Lieksa and Juuka in North Karelia, close to the Russian border.
But the Finnish Meteorological Institute (FMI) says that a cooler weather front has already arrived in western Finland and will start moving eastwards soon, making the current heatwave quite short-lived.
More storms thunder in
Severe thunderstorms are expected to whip up very strong wind gusts on Monday, warns the FMI.
Winds may reach speeds of 25 metres per second in North Ostrobothnia and western Finland, and elsewhere in the country gusts of some 15 m/s are expected.
Additionally, heavy rainfall is forecast for various parts of Finland on Monday.
Thunderstorms left thousands without electricity in different parts of Finland on Sunday. Most of them had been fixed by Monday afternoon. At 1pm, some 200 Fortum customers were still without power. Elenia (formerly known as Vattenfall) had about 1,200 customers without service at that time.