Bogota is a disorienting riot of sound, where salsa music blasts from buses, street vendors hawk their wares, car horns blare unsparingly in traffic and people everywhere clutch radios to their ears, the volume way up high.

A tremendous loneliness accompanies this noise, as eight million people strive to survive in an atmosphere of distrust and insecurity.

On board the buses, a parallel economy flourishes. In the space of a half-hour journey, an Andean flute band, an ex-convict and a poet paraded before me: "I'm standing here before you because I got out of jail today," said one shabby-looking individual with a nasty wound carved into his forehead. "I'm asking you for money not because I'm lazy but because I can't get a job and I don't want to return to thieving for a living."

The passengers shifted uneasily in their seats, digging for coins, thinking perhaps of the unprotected homes they had just left behind.

It's only after dark, walking alone in the city that you notice something odd - silence. Once the evening rush is over, the buses become scarce, the footsteps less frequent, the shadows more hostile.

While weekdays blend into one, Sundays are dedicated to the pursuit of leisure. Last weekend the typical grey sky gave way to warm sunshine, as homeless kids shook themselves awake, casting aside cardboard "blankets", carefully folded and put away for the next evening.

The homeless surrender their sleeping spaces to armed guards who watch over the shop entrances by day, carrying pump-action shotguns. These unwieldy weapons are great for exterminating large jungle animals at 200 metres but less effective against a thief running away among hundreds of shoppers.

The old-fashioned Florida cafe opens its doors, offering hot chocolate and doorstep-size bread rolls to early risers. Traffic tyranny is ended for a whole seven hours each Sunday, freeing up Bogota's principal artery, Seventh Avenue.

Little by little, the abandoned highway fills up with cyclists, joggers, wheelchairs and rollerblades, as entire families set out for the day carrying backpacks with dry corn arepas and fizzy Coke bottles.

By midday, on the corner of 36th street and Seventh, hundreds of people are loosening up to dance music under the direction of four aerobics teachers, who smile and motivate this madcap orchestra on speed.

Bogota's last mayor, a mathematician called Antanas Mockus, focused his energy on restoring civility to the city. Mime artists were hired to mock jaywalkers, all clubs closed by 1 a.m. and free rock concerts were held to entertain idle youth in city parks.

The weekend flea market is packed with casual shoppers, while overflow stalls set up illegally on the pavement outside. If you want a needle for an obsolete record player, or an alpaca jumper for a fiver, then you've come to the right place.

A police truck pulled up and a dozen officers, batons drawn, kicked the overflow merchandise and hustled street vendors into the back of a truck. It's the casual brutality of the encounter which shocks the most, the state pummelling the vulnerable with impunity.

When I opened the paper that day there was a page-long account of a young man who died in police custody from a collapsed liver. His friends saw him carried away by police after an inconsequential punch-up. Witnesses inside the police station heard the screams of the youth each time an officer came and went. The police report insisted that the detainee killed himself by "repeated blows to his own body".

The sad spectacle of the city's street life gave way to the comfortable surroundings of the Patio restaurant, one of Bogota's finest, where wood carvings, candles and cushioned armchairs insulate visitors from the uncertainty beyond. Manuel Alvarez, the country's leading playwright, discussed his film-making plans over the delicious Greek and Italian food.

As he drove me back to the hotel, he broke several red lights, while I sank deeper into the passenger seat, trying to become invisible.

Days later, a mutual friend explained the context of his loutish driving. One night Manuel and a friend stopped at a traffic light. His BMW was carjacked and five armed men took them to a lonely patch of waste ground, blindfolded them, put guns to their heads and doused them in petrol.

The city is sharply divided along class lines, with the haves living north of 50th street, where garbage-strewn side streets gradually give way to manicured lawns and shopping malls, cybercafes and Harley Davidson restaurants.

Most of Bogota's inhabitants seem to have lost any connection they had with the city. Teresa Valdes, an investment broker and old friend, once sang the city's praises and revelled in the noisy bustle. She was robbed three times last year, her daughter held captive on one occasion until an account was emptied by cash card.

All she wants to do now is leave the country. She has already bought a ticket for her daughter, who leaves this week. Opinion polls show her desire is shared by 90 per cent of the nation.