'Indecent' Zuma painting difficult to ban: judge

South African President Jacob Zuma's bid to ban a painting in which his genitals are exposed on Thursday suffered a setback with a court saying it would not be easy to enforce any order.

Satirical artist Brett Murray's depiction of Zuma in a Vladimir Lenin pose -- branded "indecent" by Zuma's ruling African National Congress -- was vandalised on Tuesday amid a national furore.

The party on Thursday went ahead with a court application against the artist, the Goodman Gallery in Johannesburg that displayed it and the City Press newspaper which published a picture, despite the gallery closing and removing the work.

But judges were not convinced that a ban would work.

"This image is already out there on the Internet," said Judge Fayeeza Kathree-Setiloane.

If a permanent interdict was granted that forbade the display of the painting or copies of it, "how will this court monitor compliance?" she asked a packed court that included several of Zuma's children and ANC secretary general Gwede Mantashe.

Opponents say the artist's depiction of Zuma is racist and an insult to the president, while supporters justify the painting on grounds of freedom of expression protected by the country's constitution.

The Goodman Gallery closed indefinitely on Tuesday after two protesters smeared red and black paint over parts of the painting showing Zuma's face and genitals. The pair appeared in court of Wednesday.

Outside the court a few hundred ANC supporters wearing the party's green, gold and black colours sang and danced, while the trial was broadcast on a big screen.

Police cordoned off roads around the building and former ANC armed wing soldiers and riot police guarded the entrance.