A police officer uses pepper spray during a protest against President Dilma Rousseff's appointment of former President Luiz Inacio Lula da Silva as her chief of staff, in front of the Brazilian national congress in Brasilia, Brazil, March 17, 2016. [Photo/Agencies]

BRASILIA - Former Brazilian President Luiz Inacio Lula da Silva was sworn in as chief of staff to his successor Dilma Rousseff on Thursday as a judge sought to block his appointment and Congress began proceedings to impeach her amid the country's deepening crisis.

Police used tear gas to disperse hundreds of opposition demonstrators who clashed with Lula's leftist supporters outside the presidential palace where he was sworn in, while ministers and corruption investigators traded barbs throughout the day.

Demonstrations also blocked major avenues in Sao Paulo and Rio de Janeiro, spurred by anger that Lula's appointment will shield the former president from prosecutors who charged him with money laundering and fraud as part of a sweeping graft probe centered on state-run oil company Petrobras.