CAPE TOWN (AFP) - A giant billboard calling for President Jacob Zuma to be ousted appeared in central Cape Town on Friday, startling the normally sleepy city into feverish speculation over who was behind it.

The white billboard, covering six storeys of an apartment block, read simply: "ZUMA MUST FALL".

Twitter lit up with speculation over suspects, with perhaps the most trenchant suggestion being that it was "Someone with big balls & deep pockets".

The main opposition party, the Democratic Alliance, which has repeatedly called for Zuma's impeachment, denied it was responsible.

Cape Town authorities said they would prosecute the culprits because the billboard did not comply with local by-laws.

The "must fall" call has become widely popular in South Africa for anything that is disliked, after being coined last year by students demanding the removal of a statue of British coloniser Cecil Rhodes from the University of Cape Town.

The students won that battle, and the statue was taken down.

While many Twitter users hailed the billboard as "free speech", some suspected that white South Africans were behind it.

"Racism is older than #Zuma so it must fall first," wrote Do-me Mhlongo @TheRealDo-me.

Racial division remains a major issue in South Africa, 21 years after the end of apartheid.

Calls for Zuma to resign or be kicked out of office are not new, with accusations that he presides over growing corruption, unemployment and a struggling economy.

But they became more vociferous last month after he sacked two finance minister within four days, sending the rand currency to historic international lows.

One man who might find the billboard embarassing is Deputy President Cyril Ramaphosa, who is tipped as a candidate to succeed Zuma.

The billboard hangs directly above a McDonald's fast food outlet - and Ramaphosa is the controlling shareholder in South Africa.