CARACAS/ SAN CRISTOBAL (Reuters) - At least a dozen protesters arrested in Venezuela remained in jail on Monday and masked assailants burned a bus amid scattered unrest over swelling lines for basic goods, activists said.

Police rounded up 16 people for protesting outside stores over the weekend, according to the opposition MUD coalition, which said four of them were released shortly after.

Rights group Penal Forum said 18 protesters were still behind bars on Monday. The government did not confirm that.

Venezuela is suffering from chronic shortages of goods ranging from diapers to flour that have worsened since an ebb in deliveries over Christmas. The scarcity has forced shoppers across Venezuela to line up in front of stores before dawn.

The MUD also accused soldiers posted outside shops of banning photos of the lines, which can snake around blocks.

"Not only is the government forcing people to get into humiliating queues ... it also wants the lines to be Cuban-style, silent and terrified," said MUD chief Jesus Torrealba.

On Saturday, an explosive device was thrown into a building of the state phone company Cantv in southeastern Puerto Ordaz city, burning eight vehicles, the government said. In western San Cristobal, six masked men threw a Molotov cocktail into a parked bus belonging to a university, students said on Monday.

National Assembly President Diosdado Cabello on Monday condemned what he called a strategy by enemies of the revolution to foment unrest in queues and called on Venezuelans to resist "provocations."

While the scattered unrest is a far cry from massive demonstrations that rocked the country for four months in 2014, it comes amid growing frustration over the economic crisis.

President Nicolas Maduro, whose popularity has plunged, says right-wing agitators and Venezuela's elite are trying to topple him via an "economic war."

"At the start of this year the parasitical oligarchy ambushed us but we and the people are responding," he said at the weekend from Saudi Arabia.

In the last week, Maduro and his closest ministers have visited China, Russia and other oil producing countries to seek financing and OPEC action on tumbling oil prices.

"This is an emergency, it's not the time for photos of Maduro doing tourism in China," said Henrique Capriles, who narrowly lost a presidential election to Maduro in 2013.

"I think it's time for our people to protest in the street."

Critics blame Venezuela's recession on socialist policies such as 12-year-old exchange control system, one that fails to provide enough hard currency for imports.

State-run supermarkets have started restricting access based on identity cards. Only Venezuelans whose card number ends in 0 or 1 were allowed to shop on Monday, local media reported.