The previously announced operation began at 5:00 a.m. when armored cars and tanks rolled into the main “favelas,” or shanty towns, that make up the complex, home to roughly 120,000 people.
After taking up their positions, the army soldiers and marines who will be in charge of security in Mare through July began patrolling different areas of the complex in search of drugs and weapons.
Authorities have not yet provided information on arrests or drug seizures.
A total of 2,050 army soldiers, 450 marines and 200 military police took part in the operation.
Launched 68 days before the 2014 World Cup kicks off in Brazil, the operation will initially last until July 31, although army General Ronaldo Lundgren said the military presence in the complex could be extended further if authorities deem it to be necessary.
Nearly 1,200 Brazilian police, supported by armoured vehicles, occupied Mare last week in an initial stage, but the elite military troops began replacing them Saturday.
Police paved the way for the occupation in recent weeks by maintaining a constant presence on roads leading into the complex and conducting operations in other Rio favelas.
Those operations have left 16 dead, eight injured and around 160 arrested in recent weeks, according to the Rio de Janeiro state government.
Rio de Janeiro’s Maracana Stadium will host several World Cup matches, including the July 13 final.