Putting together details now: preliminary reports are of armed attackers (automatic rifles, grenades). Some indication that Westerners are being targeted.
More as it comes in.
UPDATE: Via AoSHQ:
Unknown terrorists have conducted a coordinated attack across Mumbai's major tourist hubs, with over a dozen deaths and many injuries reported.
Gunmen have targeted luxury hotels, a popular tourist attraction and a crowded train station in at least seven attacks across India's financial capital.
Terrorists are reportedly still holed up at in three places, including Oberoi and Taj hotels.
The British Foreign and Commonwealth Office (FCO) are advising all Britons in Mumbai to stay indoors.
The FCO also report a further explosion near Mumbai airport.
The death toll is unknown. Officials have said at least 25 people have been wounded.
Reuters are reporting at least 18 fatalities.
The Times of India are reporting as many as 80 people killed and 250 injured in the attacks.
Everything further under the fold.UPDATE, 2:41 PM:
Reuters: Multiple attacks in Mumbai
As the above shows, we're still getting details about the attacks; no indication yet of the precise motivations of the attackers, although many people - including myself, frankly - are prepared to make a guess. When we hear more, we'll post it; in the meantime, our prayers and good wishes to the victims, their families and loved ones, and our ally India in general. I'm certain that the United States will provide whatever help is requested.
UPDATE, 3:11 PM:
The preliminary suspect is Lashkar-e-Taiba, a Pakistan-based Islamist terrorist group. Everybody who is surprised, raise your hands. Not AQ, but has worked with them; banned by Pakistan, but still operating in Kashmir.
Update - 5:18 PM (by absentee) Reports now indicate a relatively unknown jihadist group named http://in.reuters.com/article/topNews/idINIndia-36721020081126 has claimed responsibility.