Two Indonesian soldiers were injured after a blast yesterday at the National Monument park, near the presidential palace in central Jakarta, police said.
A spokesman said President Joko Widodo was not in the palace at the time.
"Initial findings from our investigation... found that the explosion was caused by a smoke grenade," Jakarta police chief Gatot Eddy Pramono told a news conference, which was televised live.
"We are investigating. We did not find anything else after combing the area," he added.
Inspector-General Gatot said the two soldiers are being treated at an army hospital in Jakarta. One had hand injuries while the other had a leg injury.
Explaining the chronology of events, Jakarta military chief Eko Margiyono said the two soldiers were going through their morning exercise routine when they spotted a suspicious-looking object.
"One of the soldiers held the grenade with his left hand, injuring the hand quite badly. The two men are conscious and being treated in hospital," he added.
Jakarta police spokesman Yusri Yunus said smoke grenades are usually used to disperse crowds during rallies. However, he did not link yesterday's grenade to a rally by conservative Muslims on Monday.
Within three hours, the authorities had reopened the park area where the blast took place, across the street from the headquarters of Indonesia's home ministry.
Thousands of Indonesian Muslims from conservative groups had held a peaceful rally near the National Monument park on Monday.
Among the protest organisers was the Alumni 212 movement, which was behind big rallies held in 2016 to demand action against former Jakarta governor Basuki Tjahaja Purnama, a Chinese Christian eventually jailed for blasphemy in a case that drew international condemnation.
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 in Iraq and Syria-inspired Jamaah Ansharut Daulah, which is responsible for a series of attacks across the country.
In October, a suspected Islamist stabbed former chief security minister Wiranto at a function. Mr Wiranto, who uses just one name, like many Indonesians, had to have surgery but is no longer in hospital.