By Jemima Kelly and Huw Jones

LONDON (Reuters) - The World Wide Web needs a complete rethink to prevent spying and the spread of "nasty, mean ideas" on social media websites, its inventor, Tim Berners-Lee, said on Monday.

Berners-Lee, a London-born computer scientist who invented the Web as a platform on top of the internet in 1989, said his intention in building it had been for the public to "do good stuff" and share ideas among each other, as was the case with websites such as Wikipedia.

Instead, negative ideas were proliferating on social media sites in particular, he said, while privacy was also being compromised by online spying.

"We need to rethink the way we build society on top of these web pages," he told the Innovate Finance global fintech summit in London.

"How come nasty, mean ideas, seem to have travelled more prevalently than constructive ideas on Twitter sometimes? Is that the way it has been designed? Could Twitter be tweaked?"

Social media has become active in polarised political campaigns such as the U.S. presidential elections and Britain's referendum on membership of the European Union last year.

There have also been instances of public figures being abused online, often by robots programmed to send out negative tweets.

The conclusion was that a "complete change of strategy" was needed. Facebook and Twitter were already rethinking approaches, he said.

Berners-Lee, who has previously criticised state-sponsored eavesdropping as well as censorship, said he had given humanity "an open internet to play with" in the hope that they would use it in a positive manner.

"We have tried to keep it open, we kept it royalty-free. We have kept it open in the sense of no censorship. On a good day, in a good country, we keep it free of spying."

There was a need to analyse the effects networks have on society, Berners-Lee said.

"We actually have to not leave people to make whatever social networks they like."

Last week he told the Guardian newspaper that U.S. President Trump administration’s decision to allow internet service providers to sign away their customers' privacy and sell customers' browsing habits was "disgusting", after he won the prestigious Association for Computing Machinery’s AM Turing award.

Despite the web being world-wide, with the possibility of school children from India, China and Syria interacting with each other, Berners-Lee said, people were broadly parochial, choosing to communicate with others like themselves.

(Editing by Pritha Sarkar)