Caracas (AFP) - A teenage boy and a woman died after being shot Wednesday during massive protests in Venezuela against President Nicolas Maduro, bringing to seven the number killed this month in a mounting political crisis.

Riot police fired tear gas to force back stone-throwing demonstrators as tens of thousands of people joined protest marches in Caracas and several other cities.

The opposition has accused Maduro of letting state forces and gangs of armed thugs violently repress demonstrators as he resists opposition pressure for him to quit.

Despite Wednesday's deadly violence, his opponents upped the ante by calling for fresh protests on Thursday.

"Tomorrow at the same time we are summoning the whole Venezuelan people to mobilize," senior opposition leader Henrique Capriles told a news conference.

"Today there were millions of us and tomorrow even more of us have to come out."

- Shot in head -

The 17-year-old boy was shot by gunmen on motorbikes who also threw tear gas canisters into a crowd of protesters, Amadeo Leiva, head of the Clinicas Caracas Hospital which treated him, told AFP.

A 23-year-old woman, Paola Ramirez, died after being shot in the head in the western city of San Cristobal, the state prosecution service said later in a statement.

Authorities had previously reported five other people killed, including a boy of 13, in protests around the country earlier this month.

- 'Hour of combat' -

Pressure on the leftist president has been mounting since 2014, as falling prices for Venezuela's crucial oil exports have aggravated an economic crisis.

Venezuelans are suffering shortages of food and medicine.

"I don't have any food in the fridge," said protester Jean Tovar, 32, who held rocks in his hands ready to throw at military police in Caracas.

"I have a two-year-old son to support and I am unemployed, and it is all Maduro's fault."

Recent moves by Maduro to tighten his grip on power and ban Capriles from politics have escalated the country's political and economic crisis and sparked international cries of concern.

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They have galvanized the often divided opposition in its efforts to force Maduro from power.

"We have to end this dictatorship. We're fed up. We want elections to get Maduro out, because he's destroyed this country," said protester Ingrid Chacon, a 54-year-old secretary.

The president in turn has urged his supporters, the military and civilian militias to defend the socialist "Bolivarian revolution" launched by his predecessor Hugo Chavez in 1999.

"The hour of combat has arrived," Maduro said this week.

- US concern -

The center-right opposition has called for the military -- a pillar of Maduro's power -- to abandon him.

But the defense minister, General Vladimir Padrino Lopez, has pledged the army's "unconditional loyalty" to Maduro, who has accused the opposition of inciting a "coup" backed by the United States.

In Washington on Wednesday, Secretary of State Rex Tillerson said the United States is "concerned that the government of Maduro is violating its own constitution and not allowing the opposition to have their voices heard."

- Election talk -

According to a survey by polling firm Venebarometro, seven in 10 Venezuelans disapprove of Maduro, whose term does not end until 2019.

Maduro said Wednesday he was willing to face his opponents at the ballot box.

"I want to have elections soon... to seek a peaceful path so the Bolivian revolution can put the conspirators, murderers and interventionist right-wingers in their place," he told a rally of supporters in central Caracas.

Regional elections due in December were indefinitely postponed and there is still no date for local polls due this year. The next presidential election is due in December 2018.