LONDON, Aug 19 (Reuters) - WikiLeaks founder Julian Assange used the balcony of Ecuador's London embassy on Sunday to berate the United States for threatening freedom of expression and called on U.S. President Barack Obama to end what he called a witch-hunt against WikiLeaks. Speaking from the balcony of the embassy, where he is staying to avoid arrest by British police who want to extradite him to Sweden, Assange said the United States risked shunting the world into an era of journalistic oppression.