Madrid - Twenty years after internet technology was invented, the web was hailed as being vital for human progress at the 18th annual international World Wide Web Conference on Wednesday.

The four-day congress brought together about 1,000 experts in the Spanish capital Madrid.

Tim Berners-Lee, one of the inventors of the internet, said it would still continue developing for a long time.

Practices such as internet searches and advertising had already become "sciences," but many challenges remained, he said, criticizing "walled gardens" in social networking.

Other challenges included being able to check on the reliability of information and the protection of anonymity and privacy, Berners- Lee explained.

Only about 20 per cent of world population is now connected to the internet, according to experts at the congress.

In many developing countries, people are first able to log in over mobile phones, Berners-Lee said, stressing the usefulness of such connections.

The internet had become an "indispensable tool for the progress of human beings and their life in society," Spain's Crown Prince Felipe said in his opening address. (dpa)