* Ruling ANC expected to win May 7 election comfortably
* Tutu tells public not to vote like "cattle"
* S.Africa celebrates 20 years since end of apartheid Sunday
JOHANNESBURG, April 23 (Reuters) - Retired Archbishop Desmond Tutu, one of the champions of South Africa's struggle for democracy, said on Wednesday he will not vote for the ruling African National Congress (ANC) party in an election on May 7.
"I won't vote for them (ANC). I say that with a heavy heart," Tutu told reporters in Cape Town, after praising former liberators within the ANC, including the late Nelson Mandela, South Africa's first black president.
"Think! Don't vote mindlessly. Don't be voting cattle," Tutu added. "Remember, it will decide what quality of life you will have for the next five years."
South Africa will celebrate 20 years since the end of apartheid on Sunday but Tutu said the ANC under President Jacob Zuma had "failed to fill the shoes" of Mandela and his colleagues who fought for the end of white-minority rule.
As one of South Africa's leading moral lights, Tutu's swipe is likely to add to building public anger at corruption within Zuma's administration, but the ANC is still expected to comfortably win the nationwide election in two weeks.
ANC leader Zuma is expected to be re-elected by parliament for a second five-year term as president after the vote, even though several corruption scandals have blotted his reputation and turned many voters against him.
Zuma was personally criticised in an anti-graft report released last month, which said he benefited "unduly" from an "excessive" $23 million state-funded security upgrade to his home. (Reporting by Joe Brock; Editing by Angus MacSwan)