Deby, who has governed the poor, oil-rich state since 1990, said the incoming republic was “the quintessence of the people’s aspirations and expectations.”
“It requires new and virtuous attitudes, practices and behaviour,” he said.
“I readily admit that many people have built large fortunes on the basis of embezzlement and corruption,” Deby said. “My dear compatriots, I assure you that this illicit and immoral enrichment machine will stop.”
The new republic is the fourth since Chad gained independence from France in 1960.
The landlocked country has a history of political violence and corruption. It is ranked a lowly 165th out of 180 countries in Transparency International’s 2017 Corruption Perceptions Index.
The constitutional changes establish a full presidential regime without a prime minister or vice president.
In addition, the current presidential term of five years, with no limits on re-election, is replaced by a six-year term, with a limit of two terms.
The changes were passed by lawmakers on Monday despite calls from the opposition, civil society groups and the Catholic church for a referendum.
The draft approved by the legislature was a text crafted in March by a national forum of politicians, religious and social groups, but boycotted by the opposition and part of civil society.
The opposition argued that Deby’s hold on power would be strengthened and democratic safeguards weakened by the change.
Twenty-six lawmakers appealed to the constitutional court for the changes to be annulled, but this was rejected on Thursday. Prime Minister Albert Pahimi Padacke and his government resigned shortly afterwards.
Deby on Friday reiterated that legislative elections, which have been repeatedly postponed since 2015, would take place this November but said the date would be “difficult to keep” without international help.
He also declared a “general amnesty” enabling all Chadians “who for one reason or another have left the country” to return home.