* World's No. 4 gold mining company affected
* Protest marches in major cities
* Unions sending signal to ruling ANC
By Ed Stoddard
JOHANNESBURG, March 7 (Reuters) - Tens of thousands of South Africans joined a one-day national strike on Wednesday, hitting mining production, as the biggest labour group in the continent's largest economy flexed its muscles to remind the ruling ANC of its political clout.
Gold Fields, the world's No. 4 producer, said its operations had ground to a halt, with as many as 85 percent of workers downing tools in response to the call for a strike by the Congress of South African Trade Unions (COSATU).
Harmony Gold, another major producer, said all of its staff affiliated to the National Union of Mineworkers, South Africa's biggest union, had stayed away. Neither company suffered any impact to their share price.
The immediate targets of the strike are new road tolls around Johannesburg, and short-term contract labour agencies that COSATU says exploit workers and perpetuate the inequalities of apartheid white-minority rule that ended in 1994.
"Despite the political and social gains scored since 1994, the working class in this country continues to reel under the pressure of neo-liberalism and the legacy of apartheid and colonialism," COSATU said.
However, despite its official billing, analysts said the strike was just as much about COSATU sending a reminder of its clout to the ruling African National Congress (ANC), which is due to elect a new leader at the end of the year.
As things stand now, President Jacob Zuma is the clear front runner to be re-elected ANC leader, and on track to win a second five-year term as president in a 2014 election.
However, Zuma cannot afford to ignore the unions - integral players in the struggle against apartheid which wield considerable grass-roots political power inside the ANC.
"Within the confines of ANC politics, there is clearly an attempt to spin the march as a challenge to Jacob Zuma's leadership and a criticism of his commitment to workers and the poor," Cape Town-based political economist Nic Borain said.
"This kind of jostling is commonplace in the politics of the ruling alliance," he added. "This is the left-Cosatu faction reminding Zuma that its support is conditional."
"MODERN DAY SLAVERY"
Several thousand red-shirted COSATU supporters gathered in the centres of Johannesburg and Cape Town under the watchful eye of dozens of police in riot gear, backed by mounted officers, dog units and water cannon.
However, the atmosphere was good-natured, with marchers singing, chanting and waving placards saying "Labour broking equals modern day slavery" and "Stop E-tolling, it's highway robbery".
Business groups have criticised COSATU's action as political posturing that is going to pile more pressure on an economy forecast to grow at a relatively feeble 2.7 percent this year.
They have also disapproved of COSATU's targeting of contract labour agencies, saying they are essential for seasonal industries such as farming, and without them, South Africa's 24 percent unemployment rate might be even worse.
Any attempts to cut labour market red tape, in particular by making it easier to hire and fire workers, are met by fierce union opposition, but Finance Minister Pravin Gordhan tiptoed cautiously into the fray with a newspaper editorial advocating a shake-up.
"Given the scale of the unemployment challenge, no single policy offers the solution," he wrote in Business Day. "There is no panacea, no silver bullet. What is needed is a comprehensive set of reforms that maximise job creation."