An explosion of a suspected smoke grenade on Tuesday at a park near Indonesia's presidential palace in the heart of the capital injured at least two military personnel, Jakarta's police chief said.
However, President Joko Widodo was not at the palace at the time, a presidential spokesman said.
It was not immediately clear if the blast was the result of a sectarian attack, but Indonesia, the world's most populous Muslim-majority country, has suffered a resurgence in homegrown militancy in recent years.
The explosion, at 7:15 a.m.(0015 GMT) on the north side of the park, was thought to have been caused by a smoke grenade, Jakarta police chief Gatot Eddy Pramono told a televised news conference.
It injured two military personnel who were there for exercise, Jakarta military chief Eko Margiyono said, adding that they were taken to hospital and were conscious.
"A smoke grenade was found and we're looking into why it was there," Margiyono said, in comments aimed at reassuring the public that described the incident "as not so extraordinary".
Asked if the blast was the result of an attack, Pramono responded, "We haven't concluded so, because we're still investigating."
Within three hours, authorities had reopened the park area where the blast took place, across the street from the headquarters of Indonesia's home ministry.
Last month, six people were wounded after a 24-year-old university student blew himself up outside police headquarters in the Indonesian city of Medan.
The attack was linked to the Islamic State-inspired Jamaah Ansharut Daulah (JAD), responsible for a series of attacks across the country.
In October, a suspected Islamist stabbed and wounded former security minister Wiranto at a function to open a university building. Wiranto, who uses just one name, like many Indonesians, had to have surgery, but has since left hospital.