Terror suspects arrested Thursday led police to five massive bombs buried beneath a gas pipeline near a church just outside Indonesia's capital, officials said.
Djoko Suyanto, a security minister, said he believed Islamic militants had been plotting an attack ahead of Easter celebrations. The U.S. embassy urged Americans to be vigilant.
The explosives, safely defused at the scene, had been set to detonate by cell phone at around 9 a.m. Friday.
"The army and police are under high alert," Suyanto told reporters, adding that troops would be deployed at churches and other strategic locations. "We want to guarantee safety."
Indonesia, the world's most populous Muslim nation, has been battling extremists since 2002 when al-Qaida-linked militants attacked two nightclubs on Bali island, killing 202 people, many of them foreign tourists.
Several attacks since then targeted glitzy hotels, restaurants and an embassy, killing another 60. Hundreds of suspects have been arrested, convicted and thrown in jail.
In recent months, small bands of militants hoping to turn the secular nation of 237 million into an Islamic state have shifted their focus to local "enemies."
They've gone after police, members of a minority Islamic sect deemed "deviant," Christians and moderate Muslim leaders.
National Police Chief Gen. Timur Pradopo said the 19 suspects were arrested Thursday, including six accused in a series of mail bombs sent last month to liberal Muslim activists and a former anti-terror chief.
Several people were wounded in the parcel bombings, none seriously.
The arrested men eventually led police to the gas pipeline 100 meters (yards) from a Catholic church large enough to hold 3,000 people in Serpong, Pradopo said.
They discovered five bombs that together weighed 150 kilograms (330 pounds) and were rigged to be detonated by cell phone, according to Nardi Atmaja, a church official at the scene.
Earlier police had said there was just one bomb. The reason for the discrepancy was not immediately clear.
Local media quoted investigators as saying each explosion would have had a reach of up to 250 meters (yards), presumably engulfing the church in flames during Good Friday celebrations.
The U.S. embassy in Jakarta issued a statement urging Americans to be especially cautious over the weekend and to stay clear of demonstrations.
"Even demonstrations intended to be peaceful can turn confrontational and possibly escalate into violence," it said.
Ninety percent of Indonesians are Muslim, though most practice a moderate form of the faith and abhor violence. A small, extremist fringe has become more vocal, and violent, in recent years.
Associated Press writer Niniek Karmini contributed to this report from Jakarta.