The first terrorist attack since 2005 targeted Jakarta's Ritz Carlton and JW Marriott Hotels.
The attacks killed at least nine people, and wounded more than 50, scattering debris and glass onto the streets, police said.
One of the blasts hit a monthly breakfast hosted by an American consultant to bring together foreign business executives and Indonesian officials. Among the wounded executives were officials from the US mining company Freeport McMoRan Copper and Gold Inc. and Holcim, the second-largest cement company in the world.
"I heard a shudder like something heavy being dropped," says David Norris, a Freeport employee who was staying at the Ritz Carlton but was not attending the breakfast. "I went to the window and saw smoke billowing up the side of the building."
An Indonesian police officer who spoke on condition of anonymity said authorities were investigating a range of scenarios for the bomb blasts Friday, but that it bore the "hallmarks" of the Jemaah Islamiyah.