By Alex Kirby BBC News Online environment correspondent

On day 10 of Alex Kirby's diary from the World Summit on Sustainable Development in Johannesburg he finds the police are worth a story in themselves.

If the Johannesburg summit has any unsung heroes, I think many visitors would vote for the police.

Sometimes the police image is chintzy, almost mumsy

Twenty years ago, 10 even, it would have been an incredible claim to make.

But these last two weeks the police service of South Africa, as it now calls itself, has worn a smile.

We don't get any hassle from the officers at the security checkpoints - on the contrary, they remain patient and friendly all day long.

I am fascinated, though, by the way almost every police officer you see wears a badge inscribed "Inspector".

I did a double take today on spotting my first plain unadorned constable.

Mumsy image

I have been intrigued by the two-man police cycle units (and men are the only ones I have seen).

I talked to one, resplendent in his fluorescent jacket and shorts, with thighs as thick as my waist (which is saying something).

We have been privileged during the summit, we have seen a very appealing South Africa... I fancy we may have been lucky, though

Astride his state-of-the-art mountain bike, he said he had volunteered for the job - and it did keep him fit.

Sometimes the police image is chintzy, almost mumsy.

They welcomed the UK Prime Minister, Tony Blair, on a visit to Alexandra township's police station with plates of biscuits and a china pot of tea.

Talk to South Africans of all races, though, and it is clear that the service does not consist solely of laughing policemen who model themselves on Dixon of Dock Green, the lovable and not very realistic copper of British post-war television fame.

Unmistakeable message

One resident told me how a friend of his had been planning to make a complaint against officers at the local police station, but was warned not to by the station commander.

"They know where you live", he said. "It really wouldn't be a good idea to take it any further."

Others say corruption is common, as underpaid officers try to extort enough to make a living wage.

We have been privileged during the summit, we have seen a very appealing South Africa.

I fancy we may have been lucky, though.

The police kept their cool the other day, when protesters marched from Alexandra to the summit venue to voice their demands (and the protesters kept their cool too, some fiery placards and speeches notwithstanding.)

But the police were there in huge numbers, armed to the gills, surveying the march from behind the machineguns mounted in the turrets of their armoured cars.

The message was unmistakeable: mess with us at your peril. We are a service with overwhelming force.