Ramaphosa is one of the main candidates to succeed Zuma as head of the ruling African National Congress (ANC).
Ramaphosa is one of the main candidates to succeed Zuma as head of the ruling African National Congress (ANC) ahead of general elections expected in 2019.
For some months he has been making attacks on the president, implicated in a series of corruption scandals, notably after a controversial reshuffle at the end of March which ousted respected finance minister Pravin Gordhan.
"If the ANC is to recover its leadership role in society, then it is absolutely imperative that we act with urgency and purpose and make sure that we never become a mafia state," he said.
"Because once we become a mafia state all the wheels have come off," he added in a televised speech in Rustenburg in the north of the country, calling for the establishment of an independent investigation.
Zuma's sacking of Gordhan last month fanned public anger over government graft scandals, record unemployment and slowing economic growth.
It also led to two ratings agencies downgrading South Africa and triggered protests by tens of thousands of people on the streets calling for Zuma's ouster.
The opposition has tabled a new no confidence motion against Zuma in parliament, which is expected to be debated in the coming weeks.
"South Africa may be inches away from a mafia state from which there could be no return," South African Council of Churches general-secretary Bishop Malusi Mpumlwana warned last week.
The ANC is due to elect a successor to Zuma in December, ahead of general elections in 2019.
In power since the official end of apartheid in 1994, the party of Nelson Mandela has lost some of its popularity and suffered setbacks at local elections last year, losing control of several cities, including Johannesburg and Pretoria.