By Alex Kirby BBC News Online environment correspondent

It was a lovely day for a demo - clear skies, a cool start which warmed up to about 25C, and a lot of people determined to make their point peacefully.

They were a rag-tag army as they set out from Alexandra township to walk the eight kilometres (five miles) to Sandton.

We're here to protest about them privatising our electricity

Elderly South African woman

They were protesting about almost everything under the sun, it seemed - the privatisation of water and electricity in South Africa, landlessness in Latin America, the environmental policies of the US, President Mugabe's government in Zimbabwe.

Some banners were simple: "Stop evictions - fix our flats", read one. Others ranged from "Down with capitalism" to "Hands off Iraq".

Range of causes

There was a big pro-Palestinian section of the march, cries of "Amandla! Intifada!" mingling South Africa's own freedom struggle with the Middle East.

There was a brutal directness about a few: "Osama, bomb Sandton", and "Mbeki, you bloody capitalist" were some of the milder ones.

Many protesters wore the distinctive T-shirts of the Anti-Privatisation Forum

I thought at one point it was going to be a rough day when an announcement came from the organisers' truck: "Comrade media, comrade press, your time is about to expire."

But it was only a warning to crews and snappers to dismount, so the march could start.

In a place of honour on the truck rode a hero of the anti-apartheid struggle, Dennis Brutus, introduced as "an ancient lion".

He was shot by the South African police in 1962, and spent years in exile.

Good-natured protest

Off we went, the marchers mostly black, mostly clad in the red T-shirts of the Anti-Privatisation Forum (APF) and the Landless People's Movement.

But there were people of all colours, from across the world, and from across the spectrum of age.

If the ANC Government don't deal with the poor and the landless, we'll do the same to them as we did to the apartheid government

John Apollis, Anti-Privatisation Forum

Two old South African women I spoke to were in their mid-seventies, one walking with a stick.

"We're here to protest about them privatising our electricity," one told me. "And we will protest."

Further along the route, though, there were buses for the elderly.

It was all good-natured as we walked through the unkempt streets of the township, past the tiny shacks where some are forced to live, the fetid pools of sewage and rubbish, and large crowds of bystanders, some supportive, most just looking bemused.

Heavy policing

The traditional "carnival atmosphere" then? No. There was a highly discordant note - a very heavy police presence throughout the length of the route.

Most were armed, with batons, sidearms, shotguns or stun grenades, and sometimes a combination.

A sight to amaze those who fought against apartheid were the hated Caspars, huge armoured vehicles topped by steel-helmeted soldiers manning machine guns.

The Caspars used to roar into the townships and terrify the inhabitants at will. It looked as if they were ready to do exactly the same again.

Soldiers and police watched the protesters' every move

Eventually the march reached an underpass beneath one of the city's many motorways. The underpass divides Alexandra from Sandton, and the contrast was immediate.

Dusty roads and huts crowded hugger-mugger gave way to elegant business centres and residential blocks, sheltering behind iron gates, high walls and electric fences.

There were trees - most in Alexandra have been chopped down for fuel.

Getting fiery

Three and a half hot hours after setting off, we reached the conference centre, protected by still more police, with water cannon, razor wire, and horses clad in visors and shin pads.

It might have turned nasty here. There were some fiery speeches.

John Apollis from the APF set the crowd roaring when he announced: "If the ANC Government don't deal with the poor and the landless, we'll do the same to them as we did to the apartheid government".

Dennis Brutus said something he presumably never thought he would have occasion to say again: "We will continue to struggle until we are free."

The speeches would not have sounded out of place in the Britain of 30 years ago. And perhaps that was what saved the day - as speaker vied with speaker to denounce imperialism, capitalism and the rest, the crowd gradually fell silent and began to trickle away.

One woman, Monica Mabanga, was carrying a large model shark on a pole. She told me the shark was President Mbeki: "We want him to see how his policies are hurting us. He's selling us. But I'm thankful to the police, because they stayed peaceful."

A policeman overhearing this, perhaps thankful the end was in sight, pointed to Monica and said: "You know she's Mrs Mbeki, don't you?"

It can't have been a very good day for the president. The country's international visitors saw how people still live in places like Alexandra, and how the security forces are prepared to maintain control.

One late result: Anti-Privatisation Forum 5, ANC Government 1 (own goal).