By Jack Stubbs and Brian Love LONDON/PARIS (Reuters) – Taxi drivers sowed traffic chaos in Europes top cities on Wednesday by mounting one of the biggest ever protests against Uber, a U.S. car service which allows people to summon rides at the touch of a button. Drivers of hundreds of Londons black taxis snarled traffic in the streets around Trafalgar Square, hooting their horns as they passed Downing Street, the home of Prime Minister David Cameron, and the Houses of Parliament. In Paris, taxi drivers slowed traffic on major arteries into the city center during the morning commute. San Francisco-based Uber Technologies Inc., valued last week at $18.2 billion just four years since its 2010 launch, has touched a raw nerve by bringing home the threat of technological advances to one of the worlds most visible trades.