They occupied the street in front of the MK building providing only a lane for public bus service, causing traffic snarls up to the Indosat roundabout.
The General Elections Commission (KPU) on July 22 had declared the Joko Widodo-Jusuf Kalla pair as winner of Indonesias presidential election after securing 53.15 percent of nearly 130 million votes.
In protest against the KPU decision, the Prabowo-Hatta pair had filed a lawsuit with the MK citing massive, structural, and systematic fraud.
"MK must ensure justice. We demand not only victory but also justice," an orator said on a pick-up truck to rally supporters.
The Jakarta Metropolitan Police Command has fielded 2,000 of its personnel to ensure security.
"To secure the court session today, around 2,000 personnel have been deployed," command spokesman Senior Commissioner Rikwanto said.
Police have carried out a multitiered security system and checked every visitor to the court using a metal detector.
They have also kept water cannons on standby and installed barbed wires around the courts compound.
The MK on Monday heard testimonies from regional KPU officials. MK is expected to announce its verdict on August 21.
(Reported by Joko Susilo/Uu.H-YH/INE/KR-BSR/F001)