Burundi?s President Pierre Nkurunziza
The BBC?s Prime Ndikumagenge in Burundi says the ceremony has come as a surprise as Mr Nkurunziza had been expected to be sworn in next week.
At least 100 people have been killed since Mr Nkurunziza announced in April he would run for a third term.
The government accuses the opposition, which says the third term is illegal, of provoking violent protests.
The African Union, European Union and the US State Department have all expressed concerns that July?s election was not free and fair.