President Lenin Moreno said he imposed the measure to “ensure citizens’ security and avoid chaos”.
Protesters were angered by Mr Moreno’s decision to end 40-year-old subsidies for fuel, which he said were no longer affordable.
Diesel and petrol prices are expected to more than double.
The fuel subsidies cost the government $1.3bn (£1bn) annually.
On Thursday students and the transport sector led a national strike.
Taxi, bus and truck drivers blocked roads and bridges in the capital Quito and the city of Guayaquil, bringing transport to a standstill.
Some protesters clashed with police, who fired tear gas to disperse crowds. Interior Minister Maria Paula Romo said 19 people had been arrested.
“The lack of transport affects us all, but equally the rise in gasoline prices will affect us,” Quito-based business owner Cesar Lopez told Reuters news agency.
Some protesters burned tyres and threw objects at armoured police vans, drawing criticism from Mr Moreno.
The president, who took office in 2017, said he would not allow protesters to “impose chaos”.
Under the emergency measure, Mr Moreno’s government can restrict freedoms and deploy the armed forces to maintain order.
The elimination of the fuel subsidies, introduced in the 1970s, are part of Mr Moreno’s plan to shore up Ecuador’s flagging economy and ease its debt burden.
Ecuador’s government has agreed to cut public spending as part of a loan deal with the International Monetary Fund (IMF).
The agreement, signed in March, allows Ecuador to borrow $4.2bn (£3.4bn).
On Tuesday, Ecuador announced it was leaving the Organization of the Petroleum Exporting Countries (Opec) to pump more oil and raise revenues.