Petrol prices rose from $1.24 a litre to $3.31, with diesel up from $1.36 a litre to $3.11 starting Sunday - one of the highest pump prices in the world.
Mnangagwa, who took over from long-time leader Robert Mugabe and won a disputed election in July, also announced a package of measures to help state workers after strikes by doctors and teachers over pay.
But many Zimbabweans criticised the move, worrying a knock-on spike in other costs would worsen an already hard economic situation and trigger protests and strikes. Everyone was astounded by the rate of increase and the reason attached to this, as we are at a period where, globally, fuel prices are on a steady and healthy decline. Transport costs have shot up and it simply means to have sugar coming here (shop) it will be at a higher cost.
The ZCTU said it planned the national strike from Monday in protest at the "insensitive and provocative" fuel price increase, although such calls have in the past not been widely followed.
The announcement came after fuel shortages which began in October past year worsened in recent weeks with motorists sometimes spending nights in queues at fuel pumps stretching for kilometres. "Workers' salaries have been reduced to nothing and our suffering elevated to another level", it said.
The main opposition party, the Movement for Democratic Change, issued a call for calm and said it was "in solidarity with peaceful citizen action across the country today".
"Is there anyone truly objective and rational who can sensibly argue that there has been any meaningful change in Zimbabwe since the 2017 coup? They can bring their tear gas and police but we're here to fight for this country; I'm not going anywhere", he said.
"You can't have a country where people sleep in cars for days for a commodity that should be readily available".
Mnangagwa attributed the increases to persistent shortfalls in the fuel market, largely blaming rampant illegal currency and fuel trading activities.
But he warned that the government would come down hard on "elements bent on taking advantage of the current fuel shortages to cause and sponsor unrest and instability in the country". "These prices are predicated on the ruling official exchange rate of 1:1 between the bond note and the U.S. dollar and also on the need to keep fuel retailers viable".
"To curb continued misuse of fuel in the country, Government, through relevant departments which include its security structures, have started on a comprehensive audit of all fuel draw-downs with a view to establishing points of leakages", he said.
Government doctors went on a 40-day strike beginning in early December demanding salaries in USA dollars and improved working conditions, while teachers' unions called a strike this week for better pay but their calls went largely unheeded.
Most service stations still had no fuel to sell to motorists who have been sleeping in their vehicles to queue.
The acute shortage of United States dollars has made it hard for Mnangagwa's government to import not only fuel but also medicine and other goods.
Mnangagwa has said his government would not let businesses raise prices but they have been doing so anyway, arguing that they have no choice but to buy dollars at a premium on the black market.
"The intention is to create a constant supply of fuel for diplomats and tourists to manage the country's image", said Mugano.
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