By Tanisha Heiberg and Tiisetso Motsoeneng

JOHANNESBURG (Reuters) - When the acting boss of South Africa's public broadcaster stepped down this month, he said he had deliberately limited reporting on one of the main opposition parties.

Jimi Matthews' comments added to concerns among opposition politicians, activists and the public that the South African Broadcasting Corporation (SABC) was being used as a state propaganda tool less than four weeks from local elections that are expected to be the most closely-contested in two decades.

The SABC denies Matthews' assertions, while the ruling African National Congress (ANC) denies seeking to influence the broadcaster's coverage.

The broadcaster, the nation's primary source of news, has its own independent board with a constitutional mandate to serve the public but insiders say separation from the ANC and the state only exists on paper.

Matthews, who stepped down as acting chief executive of SABC, told eNCA television earlier this month that he was under pressure to limit coverage of the militant Economic Freedom Fighters (EFF), a party formed in 2013 by the expelled ANC youth leader Julius Malema, ahead of the 2014 election.

"Certain individuals in the ruling party ... were of the view that we should not be giving Julius and his crowd coverage," said Matthews, who resigned citing a "prevailing, corrosive atmosphere" at the broadcaster.

Some journalists at SABC, which reaches more than 20 million people with 18 radio stations and four television channels, also say they have been jolted by Chief Operating Officer Hlaudi Motsoeneng's decision in May not to air footage of anti-government protesters rioting and destroying property.

Motsoeneng did not respond to repeated phone and email attempts by Reuters to contact him via the SABC.

Motsoeneng, who has pushed through a number of changes aimed at increasing local content and what he called "positive news", is considered by some political analysts to be close to President Jacob Zuma.

Zuma's popularity has been sagging as he grapples with record-high unemployment, a looming recession and a string of scandals.

SABC spokesman Kaizer Kganyago denied the broadcaster's editorial decisions were tailored to make the state, ANC and Zuma look good and defended its policy not to air footage of the violent protests.

"We are not going to show footage of people who are destroying property but we are still going to explain everything and tell people what has happened, and if that is censorship then I don't understand," he told Reuters.

The broadcaster announced the protests policy in May, saying showing the footage would encourage others to carry out similar violence.

The ruling party chief whip Jackson Mthembu last week denied the ANC was wielding undue sway over the SABC.

"We are confident if anybody were to establish an inquiry into this matter, we would be found squeaky clean," Mthembu told Reuters. 

'RESPECT ZUMA'

The broadcasting regulator, the Independent Communications Authority of South Africa (ICASA), has already ordered the SABC to reverse the decision not to air footage of violent protests.

ICASA spokesman Paseka Maleka declined to comment on any broader governance issues at SABC, saying the regulator had only scrutinized the complaint about the protests coverage, lodged by a media monitoring group.

Outbursts of violence over the lack of services such as water or roads are common in South Africa and in recent months have included the torching of schools and other property, both public and private.

The protests have taken on political significance before Aug. 3 elections, which are expected to be the ANC's sternest test at the polls since it came to power in 1994.

One SABC source said that following a management workshop held on June 7-8 this year, Motsoeneng told reporters that everyone else could be questioned in the broadcaster's news programs except Zuma because the 73-year old leader deserved to be respected, particularly at the SABC.

Reuters could not independently verify that Motsoeneng made these comments.

SABC spokesman Kganyago did not respond to repeated email and phone requests for comment from both the broadcaster and Motsoeneng about this incident.

'POSITIVE NEWS'

For the past three years the SABC has repeatedly said publicly that its bulletins should contain 70 percent "positive news", with Motsoeneng telling the Mail & Guardian newspaper in 2013 that positive stories helped build the nation.

Last week, at a news conference in which the SABC said it would challenge ICASA's ruling on banning footage of riots, Motsoeneng accused the South African media of censoring "good news".  

"Why are we not seeing good news stories? If you're not showing good stories, what do you call it? I think it is a censorship," he said.

An SABC source said journalists were struggling to put a positive spin on some items that include reporting on scandal-hit Zuma, who has fended off accusations of corruption, influence peddling and rape.

The canning of a popular talk show hours before it went on air to discuss the Guptas, a business family with close ties to Zuma, also fueled perceptions among critics that the broadcaster was being influenced by the government.

"I was summarily informed there would be no next show," show host Vuyo Mvoko said in a front-page editorial published in the Star newspaper with the headline: "My hell at the SABC". He declined to comment beyond the article.

Zuma has denied numerous allegations from opposition politicians that the Guptas wield undue influence over him. The Guptas have also repeatedly dismissed such accusations, saying they are pawns in a political plot to get Zuma out of office.

SABC has not given any reasons for cancelling the show.

(Editing by James Macharia, Anna Willard and Pravin Char)