The Taj Mahal hotel on the seafront in Mumbai was packed with foreign guests when it was stormed by armed gunmen.
Many were confined to their rooms and began to panic when the building caught fire. They had been warned they would be shot if they tried to get out, but some strung sheets together and climbed out of windows.
The Indian authorities have refused to negotiate and sent in the commandos. Those commandos are now going through the building room by room and there gunfire and explosions have been heard.
It is still not known how many people are trapped inside.
Police in Mumbai have told everybody to stay home and off the streets.
The militants attacked a total of 10 targets in Mumbai overnight on Wednesday, in a coordinated attack which the Indian government said was unprecedented.
The Cafe Leopold, one of the most popular tourist spots in the city, was the scene of another violent attack.
At the railway station it was obvious people had fled in panic. Blood smears on the floor made it obvious many people had been hurt too.
Hospitals say they are treating hundreds of casualties. Numerous western countries have confirmed their nationals are among the dead and injured.