* May 8 vote set to be most tightly contested
* Opposition parties gaining ground
* Ruling ANC to act on corruption, lift investment to $100 bln
* Opposition EFF blames ANC for economic and racial inequalities (Adds opposition Economic Freedom Fighters)
JOHANNESBURG, May 5 (Reuters) - South Africa's ruling African National Congress (ANC) will kick-start the economy and deal with corruption, it vowed on Sunday, three days before elections at which its overwhelming majority faces its sternest test since the party rose to power.
Less than 30 km (18.6 miles) away, the country's second-biggest opposition party, the Economic Freedom Fighters (EFF), challenged the ANC's governance record and promised a tougher stance on corruption and economic policies to target racial inequality.
Though the ANC has won each parliamentary election since the transition from apartheid in 1994, recent opinion polls predict that it will bleed support to opposition coalitions that have gained ground as the ANC has been dogged by political scandal and a flagging economy.
President Cyril Ramaphosa, who took over from scandal-plagued Jacob Zuma as ANC leader in December 2017, told about 70,000 supporters packed into Johannesburg's Ellis Park Stadium on Sunday that the party would punish members guilty of stealing public money.
A judicial corruption inquiry that began in 2018 has heard chilling evidence of how top officials took bribes from local and multinational companies in exchange for lucrative contracts from state firms.
"We are determined that those found guilty of corruption or involvement in state capture will not be allowed to occupy positions of responsibility, either in the ANC, in parliament or in government," Ramaphosa said at Sunday's rally.
"The era of impunity is over. We are now entering the era of accountability."
The torrent of allegations has hit ANC support, especially in urban areas.
The EFF, formed by former ANC youth leader Julius Malema in 2013, has built its agenda on highlighting state graft and championing radical economic reforms the ANC has avoided, such as mine nationalisation and land expropriation without compensation.
"The reign of thieves must fall, the reign of Ramaphosa must fall," Malema chanted at a rally of about 40,000 supporters in Johannesburg township Soweto. "The EFF doesn't have tolerance for corruption."
Three opinion polls in recent weeks show the ANC's support ranging between 51 and 61 percent, compared with the 62 percent it won in 2014.
KINGMAKER
The EFF won 10 percent of the vote in 2014 national elections. In local elections in 2016 it captured 12 percent, allowing it to play kingmaker in urban regions where neither of the top two parties managed to get past 50 percent.