* President Ramaphosa trying to halt slide in ANC support

* Jobs, land and corruption are key issues at ballot box

* First results will trickle in on Thursday morning

By Alexander Winning and Mfuneko Toyana

PRETORIA, May 9 (Reuters) - South African elections officials counted ballot papers in the early hours of Thursday, a day after a vote seen as the toughest test yet for the African National Congress (ANC) party 25 years after it swept to power at the end of white minority rule.

The elections for a new parliament and nine provincial legislatures are the first barometer of national sentiment since President Cyril Ramaphosa replaced scandal-plagued Jacob Zuma as head of state in February 2018.

Ramaphosa is trying to arrest a slide in support for the ANC, which has won every parliamentary vote since the end of apartheid in 1994 but whose image has been tarnished by corruption scandals and a weak economy in the past decade.

Opinion polls suggest the ANC will again win a majority of the parliament's 400 seats, but analysts say its margin of victory may fall amid frustration with slow progress addressing racial disparities in income and wealth.

Most polling stations closed around 1900 GMT on Wednesday.

Results from some stations will start to trickle in on Thursday morning, but a final tally could only be ready by Saturday.

Elections officials said voting had in general progressed smoothly but that there had been isolated incidents where bad weather, power outages or community protests had caused disruptions. The electoral commission said it was investigating two potential instances of double-voting.

"The Electoral Commission will not allow the potential misconduct of one or two individuals to taint the overall outcome of these elections," the commission said in a statement.

The ANC's biggest challengers at the elections are the main opposition party, the Democratic Alliance (DA), and the leftist Economic Freedom Fighters (EFF).

The ANC won 62 percent of the vote at the last parliamentary election in 2014, far short of its best result, 69 percent in 2004 under President Thabo Mbeki.

It currently controls eight of the country's nine provinces, with the DA in power in the Western Cape.

Ramaphosa, who became ANC leader in December 2017 after narrowly defeating a faction allied with Zuma, has promised to improve poor public services, create jobs and fight corruption.

But he has been constrained by divisions within his own party, where some Zuma supporters still retain influence and oppose his agenda.

(Reporting by Alexander Winning and Mfuneko Toyana; Editing by Peter Cooney)