BRUSSELS, Jan 21 (Reuters) - U. S. customs authorities will face a real challenge if President Donald Trump goes through with his threat to place tariffs on six EU countries - rather than the whole European Union - given the ease of movement of goods between EU members.

Thousands of farmers driving tractors and waving flags descended Tuesday on the European parliament in Strasbourg in protest at a major trade deal signed with South America, ahead of a vote on whether to refer it to the courts.