By Steve Scherer, David Ljunggren and Julie Gordon OTTAWA (Reuters) – Canadian Prime Minister Justin Trudeau on Thursday said he had sought emergency powers to end protests that blocked border crossings and central Ottawa because the standoff posed an economic threat that was hurting ties with the United States. Authorities have promised to clear out hundreds of truckers who have paralyzed central Ottawa, an act Trudeau had earlier called a threat to democracy. A video by a CBC reporter showed two buses of police arriving in the central core of Ottawa on Thursday morning, although there was no…