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I've reached Antigua and found an ATM that works with my MasterCard Credit Card. It's inside the Citibank in the north west corner of the main park in Antigua. Several other ATMs that were not located in banks didn't work so it's something to be aware of.
According to the Citibank Guatemala website this particular branch is located at:
4a. Avenida y 4ยช Calle esquina No. 2, Antigua, Guatemala
Approximate location on Google Maps:
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