JAKARTA - Two Indonesian soldiers were injured after a blast occurred at the National Monument park, near the presidential palace in central Jakarta, police said on Tuesday (Dec 3).

A presidential spokesman said that 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 why there was a grenade there. We did not find anything else after combing the area," he added.

Inspector-General Gatot said that the two soldiers are now being treated at an army hospital in Jakarta. One was injured on both hands, while the other was injured in his leg.

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 left hand quite badly. They are conscious and being treated in hospital," he added.

Jakarta police spokesman Yusri Yunus said smoke grenades are usually used to disperse crowd during street rallies. However, he did not link it to a rally held 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 (JAD), responsible for a series of attacks across the country.

In October, a suspected Islamist stabbed and wounded former chief 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.