The truck, parked near the ANC’s Luthuli House headquarters, was decorated with party flags and pictures of Mandela.
It was opened on one side to reveal a stage with a podium and a PA system.
Passers-by stopped to look at the truck, while people in adjacent buildings peered out of windows.
The prayer service was expected to be led by African National Congress chaplain general, Reverend Vukile Mehana.
“This forms part of the ANC’s nationwide prayer congregations, where members of the public are invited to offer prayers in support of Madiba, his family, and the medical team attending to him as he enters his fourth week in hospital,” the ANC said on Monday.
Mandela was admitted to hospital in Pretoria on 8 June with a recurring lung infection. On Monday, the presidency said his condition remained critical, but stable.
Prayer meetings have been held for the Nobel Peace Prize laureate outside the Medi-Clinic Heart hospital in Pretoria, in his Eastern Cape village of Qunu, and in other provinces.