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.
Indonesia, the world's most populous Muslim nation, was hit by a string of bombings from 2000 to 2005 attributed to the JI, an Islamist terror group that has had ties to Al Qaeda members at various times during its evolution.
Security analysts have since speculated that the quiet in recent years was because the mainstream faction of JI had turned away from high-profile attacks on Western targets as they attracted unwanted attention and reduced sympathy for the group among ordinary Indonesians.
'Control center' in room 1808
The attackers behind Friday's bombings appear to have been staying at the hotels. Indonesian police have found what they have termed a "control center" for the attack in room 1808 of the Marriott and have also released closed circuit footage from inside the hotel that appears to show a suicide attacker.
Indonesia's security minister Widodo Adi Sucipto told reporters at the scene that the blasts were caused by "high explosives," but did not elaborate. Counterterrorist officials were at the scene of the blast but said they did not want to speculate over who was responsible for the attack.
The Associated Press quoted police official Arief Wahyunadi as saying the bombs were placed in the restaurant of the Ritz Carlton and the basement of the Marriott, which was hit by a terrorist attack in 2003.
Indonesia's fight against JI
Since a series of bombings rocked eight Indonesian cities on Christmas Eve 2000, authorities in Malaysia, Singapore, and Indonesia have mounted a region-wide effort to dismantle the group and have arrested more than 400 JI members.
In Indonesia, the US-trained elite police unit Detachment 88 has hunted JI operatives across the country, capturing or killing several of its top leaders. In 2005 police shot Azahari bin Husin, JI's bombmaker. But other figures, such as recruiter Noordin Top, continue to evade authorities.
Friday's hotel bombings come close on the heels of the July 8 reelection of President Susilo Bambang Yudhoyono in the country's second direct vote, for president. Yudhoyono, who campaigned on promises of anticorruption and economic recovery, oversaw an end to a 28-year conflict with separatists in the tsunami-afflicted Aceh province and the renewal of military ties with the US in 2005. Indonesia has cooperated closely with US authorities in the campaign against terrorism in Southeast Asia.
Mr. Yudhoyono said political rivals angry at losing out in the heated and just-completed presidential campgaign may have been behind the attack, and alleged in a television interview that state intelligence has evidence that terrorists are training to murder him.
An Australian think tank in a report this week warned that the likelihood of a JI attack would increase with the scheduled release of some low-level JI operatives from jail. The Australian Strategic Policy institute (ASPI) said the released prisoners might return to violent activities.
[This story was updated from its original version at 8:48 a.m. EST]