JOHANNESBURG — A South African government watchdog group recommended on Wednesday that President Jacob Zuma’s administration be officially investigated for possible corruption.

The public protector’s office, a group charged with investigating official corruption and misconduct, said in a report that it was concerned about the apparent influence exercised by a family close to Mr. Zuma in choosing cabinet ministers. The report also raised questions about the president’s lack of interest in investigating possible corruption in his own administration.

The highly anticipated 355-page report fell short of providing clear evidence of malfeasance by Mr. Zuma, whose seven-year presidency has been marred by a series of scandals. But it is likely to increase pressure on Mr. Zuma — who is widely blamed for the poor performance of his party, the African National Congress, in local elections in August — to step down before the end of his term in 2018.

The report was released at the end of a day of large protests against Mr. Zuma in Pretoria, the capital, by the political opposition and civil society groups. The A.N.C.’s top leaders have stood behind Mr. Zuma, but there have been recent cracks in the party’s facade. Last month, the party’s chief whip, Jackson Mthembu, became the first insider to turn on Mr. Zuma, calling for the resignation of the president and other top leaders, including himself.