At an inauguration ceremony that formally extended his 32-year grip on power, Sassou Nguesso vowed to focus on boosting the economy and tackling youth unemployment.
"This five-year term will be that of the economy," he said in an address in Brazzaville attended by several other African heads of state.
"All our efforts will be on strengthening the national economy so that it is deeply diversified, dynamic and flourishing and most of all creating thousands of decent jobs."
Some 60 per cent of young people are out of work in Congo, a small but oil-rich country of some four million, according to the International Monetary Fund.
Also Read
The Constitutional Court earlier this month confirmed Sassou Nguesso's first-round win in Congo's March 20 presidential polls, with more than 60 percent of the vote. The opposition however condemned the election as a "massive fraud".
Sassou Nguesso's win only became possible after an October constitutional referendum ended a two-term limit on presidential mandates, allowing the former paratrooper to run again in a move denounced by his opponents as "a coup". Congo has been on edge ever since.
The government said at least 17 people, including two civilians, were killed in the violence, which it blamed on a "terrorist attack" by the disbanded Ninja Nsiloulou militia.
Members of the opposition however have dismissed the official version of events and accuse the government of trying to quell post-election protests.
The runner-up in the presidential election, Guy-Brice Parfait Kolelas, last week urged his supporters to accept the vote outcome despite "all sorts of irregularities" to avoid further unrest.
Sassou Nguesso first served as president from 1979 to 1992, returning to power in 1997 following a civil war.