A fire broke out early Friday morning in the entrance hall of the exhibition grounds at Prague's Letňany, fire brigades tweeted. Smoke can be seen tens of kilometers away and firefighters are asking people to keep their windows closed. The third level of alarm was sounded around 5:30 a.m. According to preliminary estimates, the damage could amount to CZK 150 million. The fire broke out in a hall on Beranových Street, where an exhibition of tents and outdoor equipment was taking place. The hall is large, measuring 250 meters in length, of which 150 meters collapsed. Firefighters tweeted earlier this morning that the fire didn't spread and that they managed to save the adjoining hall. Firefighters have four levels of alarm. The third degree is the second most serious. This is followed by a special level of alarm, which is declared if the fire threatens more than 1,000 people or entire communities. The cause of the fire is being investigated.