A homemade bomb detonated near the entrance of the Philippine justice ministry early on Monday, officials said, hours before a suspected unexploded device was found near the country's Senate.

Regional police spokeswoman Hazel Asilo declined to estimate the size of the explosion the device could have produced. (Representative Image/ Unsplash)

The explosion just after midnight, which resulted in no casualties, and the discovery of the second suspected bomb came hours before Philippine President Ferdinand Marcos was due to give an annual State of the Nation address.

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National police chief Jose Melencio Nartatez Jr told reporters on Monday there was a "big possibility or probability that the two (incidents) are related".

He said that, while police had contingency plans for "bomb threats, actual bombings and other (things)... something got past us and exploded in front of the Department of Justice".

It was not immediately clear how large the early morning blast had been.

"We are still investigating, so we cannot give you details on how massive or small the explosion was," Manila police spokesman Philipp Ines told AFP.

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Images posted by Philippine media showed what appeared to be a vehicle windshield cracked by the blast and a small amount of debris.

Police said in one report that the device at the Senate had been spotted on a street abutting the building just before 8:30 am (0030 GMT) by a tow truck driver, who alerted security staff.

Personnel trained in handling explosive ordnance removed material from the scene that included a blasting cap, clock, and a jar containing a "brown powder" for testing.

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Regional police spokeswoman Hazel Asilo declined to estimate the size of the explosion the device could have produced.

Marcos's speech on Monday comes at a time of heightened political tensions, with Vice President Sara Duterte in the middle of an impeachment trial, while voters have also been roiled by a corruption scandal tied to bogus flood control projects.