BRASILIA, Dec 29 (Reuters) - Brazilian police said they had arrested at least four people and carried out nationwide raids on Thursday in investigations into an alleged coup attempt during riots by supporters of defeated far-right President Jair Bolsonaro.Brazilian authorities, led by the Supreme Court, have been cracking down on a small but committed minority of Bolsonaro supporters who refuse to acknowledge leftist President-elect Luiz Inacio Lula da Silva's election victory and are calling for a military coup.The raids stemmed from a riot on Dec. 12, the day Lula's victory was certified, when election-deniers camped outside army headquarters in Brasilia attacked the federal police HQ and set cars and buses alight after the arrest of a pro-Bolsonaro indigenous leader.Cleo Mazzotti, who heads the federal police's organized crime division, said four people had been arrested by mid-morning, with more detentions expected as police searched for seven other suspects.George Washington Sousa, a Bolsonaro supporter with links to the army encampment, confessed to making the device to provoke the military into an intervention."