Police warned Indonesians to be on guard against future terror attacks despite a major roundup of Jemaah Islamiyah suspects and the seizure of arms and explosives.

"Threats of new attacks always exist. We have to remain vigilant," said national police spokesman Zainuri Lubis Thursday.

The country's police chief General Da'i Bachtiar has said the seizures indicated that Southeast Asian terror network Jemaah Islamiyah (JI) was planning fresh terrorist attacks before next year's general elections.

JI is blamed for the bombing of two Bali nightspots last October which killed 202 people and for dozens of other deadly attacks in recent years.

A bomb exploded early Thursday in Central Jakarta but no one was hurt and damage was slight. Bachtiar said it was designed to create fear among the public but he did not know who was responsible.

Police Wednesday announced the arrest of 18 suspected JI members, including three Bali suspects who will be flown to the resort island for questioning.

"We were looking for Bali bombing suspects and we found them. It turned out that they confessed to be members of JI and know each other," Lubis told AFP. A fourth suspect is also believed to have taken part in the Bali plot but his name could not be revealed, Lubis said.

JI's alleged "emir" or leader, Muslim cleric Abu Bakar Bashir, went on trial Wednesday for treason. Among the 18 detainees is Abu Rusdan, who according to police replaced Bashir as leader after Bashir was arrested last October. Bachtiar said reports of Rusdan's status came from detained JI members.

"Abu Bakar Bashir is facing a legal process. They might have [decided to] appoint an interim leader, maybe without a formal letter of appointment," he said.

The timing of the arrests was likely a deliberate effort to bolster the case against Bashir, said Sidney Jones of the International Crisis Group of political analysts.

"That may well have been a deliberate effort to say the JI indeed exists as an organisation and that we have additional people that may be able to provide evidence on Bashir," she told AFP.

In addition to Rusdan police say they detained a sub-regional chief of JI, a Malaysian called Nasir Abbas. "I think the police get a lot of credit because these are important figures," Jones said.

Abbas is said to be a brother-in-law of Mukhlas, who is a key suspect in the Bali bombing and one of the 30 people already detained on the island. Police said Abbas was involved in the Atrium mall bombing in Jakarta in August 2001.

Jones said she had received information late last year that even before the Bali bombings another man preferred by JI hardliners had, in practice, taken over leadership from Bashir. Bashir remained the network's official leader, she said.

The three new Bali suspects are Saad alias Ahmad Roichan, Umar Besar alias Wayan, and Sawad. Lubis said eight others are still being sought. He said police seized two handguns, ammunition and bomb ingredients from the JI suspects.

Indonesia's top detective Erwin Mappaseng was quoted by Kompas newspaper as saying that Rusdan's name emerged during investigation of the Bali bombing.

Bashir is not accused over the Bali blasts. He is charged with waging a jihad or holy war to topple the Jakarta government and set up an Islamic state. He allegedly authorised a series of bomb attacks on churches across the country on Christmas Eve 2000, which killed 19 people. He faces a 20-year jail term if convicted of treason.