Early on Saturday, the first proper day of carnaval, I was driving along the packed highway with some friends. The roads, which are normally empty, were packed with cars with case upon case upon case of beer stacked impossibly high on the roof.

The traffic jams were horrendous and to kill time I asked my friend what carnaval was really about. He told me that as carnival season opens, Brazilians start to make a sort of annual balance. It's a time to forget or recall an old love affair, to celebrate a new passion or search for new romantic experiences. It's also a time to protest against corrupt politicians, to complain about the poverty and give creative suggestions to turn the country a fair place to live in. It is also, he winked at me, a good opportunity to have the odd beer or two.

So this, I thought, is also carnaval. A time of social readjustment.

A few minutes later we were pulled over at the first of a number of police roadblocks. Normally I have no problems with the authorities here, unlike some other countries I have lived in, and it is extremely unusual to be stopped whilst driving. In fact, this was the first time it had ever happened to me.

As we climbed out the car and I was ticked off for not wearing my seat belt. I asked the policeman why we had been stopped. He told me it was nothing personal, and that it was just normal carnaval procedure to check on all cars to make sure no one is too drunk or the car is not too stolen. "After all," he told me, "anything is possible in carnaval."

We were soon on our way, and I had another definition of carnaval for my rapidly growing list: carnaval is a time when anything is possible and the authorities hold their breath and hope for the best.

Prior to 1840, the streets of Brazilian towns ran riot during the three-day period leading up to Ash Wednesday with people in masks hurling stink bombs and squirting each other with flour and strong-smelling liquids; even arson was a form of entertainment. In 1840, the Italian wife of a Rio de Janeiro hotel owner changed the carnival celebration forever by sending out invitations, hiring musicians, importing streamers and confetti, and giving a lavish masked ball. In a few years the masked ball became the fashion and the wild pranks played on the streets disappeared.

Today, Rio de Janeiro has the biggest and best-known pre-Lenten carnaval in the world - it's most colourful event is the Samba School Parade. The samba schools taking part in the parade - each roughly having three to five thousand participants - are composed overwhelmingly of poor people from the city's sprawling suburbs. Every carnaval, Rio's samba schools compete with each other and are judged on every aspect of their presentation by a jury. Each samba school must base its effort around a central theme. Sometimes the theme is an historical event or personality. Other times, it is a story or legend from Brazilian literature. The costumes must reflect the theme's historical time and place. The samba song must recount or develop it, and the huge floats must detail the theme in depth.

This year, one of the most popular schools - 5500 dancers in 47 contingents - based its presentation on the sad years of military dictatorship. Its floats recalled the victims of nearly two decades of iron fisted rule. Even the sudden, and mysterious explosion of one of the school's floats did not dampen the school's enthusiasm, and their chant of "I go from ashes to celebration" seemed to touch a collective nerve.

My own personal favourite school, Caprichosos de Pilares, based their theme on "God save Brazil". Their message was clear - only the supernatural, not economic reforms can save Brazil, and that God had best be Brazilian.

This is the glitzy, visually stunning, tourist orientated carnaval that everyone knows about, but in the North East carnaval is just as important. Towns such as Salvador and Olinda have some of the most famous, and according to a lot of people, most authentic carnavales. The main difference is that they take place on the street, whereas Rio's carnaval takes place mainly in balls and clubs which can be prohibitively expensive.

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