Lungu last week gave police increased powers of arrest and detention, alleging that opposition parties were behind a string of arson.
Lungu last week gave police increased powers of arrest and detention, alleging that opposition parties were behind a string of arson attacks intended "to create terror and panic".
The president denied he was establishing a dictatorship in Zambia -- until recently a relatively stable country -- and accused his rivals of trying to overturn last year's election results.
The emergency decree "constitutes abuse of power designed to silence his critics and kill democracy," opposition United Party for National Development (UPND) vice president Geoffrey Mwamba said in a statement.
"It is clear that (Lungu's) actions are premeditated and designed to strengthen the hand of dictatorship."
Several fires, including one that burnt down the main market in the capital Lusaka last week, have been at the centre of rising tensions in Zambia.
Mwamba denied any UPND involvement in the fires.
Zambia has enjoyed relative stability since its first multi-party election in 1991, but last year's vote was marred by clashes between rival party supporters.
UPND leader Hakainde Hichilema has also been in detention on treason charges since April.
He was arrested after his convoy allegedly refused to give way to the presidential motorcade.
Hichilema narrowly lost the 2016 election to Lungu's Patriotic Front (PF) party, and has alleged that the result was rigged.
"We insist on the speedy release" of Hichilema, Mwamba said in the statement, which was the UPND's first response to the emergency powers.
"Innocent Zambians will be arrested on mere suspicion just to fulfil the Patriotic Front's political agenda to remain in power forever," he said.
Church leaders in Zambia have also criticised the emergency powers, warning the move would scare away investors needed to boost Zambia's weak economy.
"This is a clear sign of dictatorship -- just because of a fire at a market and you declare a state of emergency?" bishop Simon Chihana, president of the International Fellowship of Christian Churches, told AFP.
"Lungu is only thinking about his continued hold on to power. He is not concerned about the well-being of Zambia."