The man caught on video holding a solo demonstration at Raffles Place last month was jailed for three weeks and fined $20,000 on Monday.
Following a two-day trial, a district court convicted Singaporean Yan Jun, 41, of four counts of taking part in a public assembly without permit and one count of disorderly behaviour.
Yan, who is unemployed, first demonstrated against Prime Minister Lee Hsien Loong and the Singapore judiciary by displaying placards in front of the US Embassy in Napier Road on Oct 20 last year.
He committed the same offence about two months later with similar placards in front of the British High Commission in Tanglin Road.
And on July 3 this year, Yan reoffended again by displaying a placard outside Raffles Place MRT station at around noon.
One side of the placard stated "PM Lee: Resign over Terrex conspiracy", while the other side had a message accusing the Supreme Court of "judicial corruption".
In November last year, nine Singapore Armed Forces Terrex infantry carrier vehicles were seized by Hong Kong Customs officials. They were returned in January this year.
He returned to the same spot at around 1pm on July 4 with a similar sign and behaved in a disorderly manner about 15 minutes later, shouting repeatedly through a loudhailer at police officers who had arrived at the scene.
Deputy Public Prosecutor Randeep Singh Koonar told District Judge Ng Peng Hong that Yan refused to stop when police told him to. The prosecution also said Yan had claimed trial despite the "overwhelming evidence" against him.
DPP Singh added: "He proffered no defence at trial, and did not even challenge any of the evidence against him. He has instead abused the trial process and used the trial as a further soapbox for him to cast baseless and spurious allegations against the judiciary, the prosecution and the police."
Yan, who was unrepresented, did not present any evidence in court in his defence.
DPP Singh urged the court to jail him for at least three weeks for disorderly behaviour and fine him the maximum $5,000 for each count of taking part in a public assembly without permit. "His sole reason for claiming trial was to use the court proceedings as a platform for airing his personal political agendas and to further malign the court and other stakeholders in the legal system. This is an egregious abuse of the court process."
Judge Ng agreed with the prosecution that Yan had shown blatant disregard for the law.
Yan said he intends to appeal against his conviction and sentence.