SAO PAULO, July 21 (Reuters) - Brazilian federal police are monitoring 100 people for possible links to terrorism groups, mostly in Brazil's tri-border region with Paraguay and Argentina, a presidential palace source said on Thursday.

The source said the 10 arrests made on Thursday for preparing acts of terror during next month's Olympics involved cooperation with U.S. and other foreign intelligence agencies. (Reporting by Anthony Boadle and Alonso Soto; Writing by Caroline Stauffer; Editing by Daniel Flynn)