As thousands of Syrian protesters poured onto the streets after midday prayers Friday, the European Union announced a ban on oil imports that could strip away a major source of financial support for the government’s five-month-old crackdown on dissent.

At least 12 protesters were killed Friday, activists said, as Syrian security forces surrounded mosques during prayers, restricting demonstrations, carrying out arrests and in some cases shooting at worshipers who chanted, “Death rather than humiliation.”

Activists and European governments hope that the ban on purchasing Syrian oil — the toughest action the European Union has yet taken against President Bashar al-Assad’s government — will make it harder to fund the effort against the protesters.

“These oil sanctions will hit the regime in the heart,” said Wissam Tarif, a human rights activist with the global advocacy group Avaaz.

“The state has made sure that ordinary Syrian citizens would feel the effects of the uprising. But we haven’t seen the regime so far signaling” that it was feeling an economic pinch itself, he said.

The United States had already banned Syrian oil imports, but that move was largely symbolic. Europe buys most of Syria’s oil, spending $4.1 billion — or more than $11 million a day — in the country in 2010, according to European Commission figures.

Syria’s economy is not transparent, but analysts estimate that before the protests started in March, oil exports accounted for 20 to 30 percent of the government’s income. Since then, with other economic sectors struggling, oil exports have become even more important.

As concern has mounted that Assad is showing no signs of relenting, the international community has become more willing to put direct pressure on his government, and activists based in the country have increased their requests for intervention. In mid-August, the United States and the European Union said that Assad needed to step down.

“President Assad is carrying out massacres in his own country,” Polish Foreign Minister Radoslaw Sikorski said Friday in Sopot, Poland, where he and his European counterparts were meeting to discuss a response to Syria.

Analysts estimate that Syria exports just over a quarter of the roughly 400,000 barrels of oil it produces every day. Because of its oil’s consistency — heavy and difficult to refine — finding other purchasers will be challenging, if not impossible, they said.

“The whole point of the sanctions is to get Bashar to borrow money,” said Andrew Tabler, a Syria expert at the Washington Institute. “If he borrows more money, he’s more accountable to other people, and he’s less likely to kill people” because he won’t be able to afford it, Tabler said.

The ban starts Saturday, but existing contracts can be fulfilled until mid-November at the insistence of Italy, where oil companies say they have already purchased oil that has not yet been delivered. The European Union has previously resisted calls for sanctions, in part because of fears they would hurt ordinary citizens.

This year’s protests in North Africa and the Middle East have affected U.S. oil supplies far less than Europe’s. Libya was a major oil exporter to Europe before its civil war started in February, and five foreign oil companies are already back and working in that country, the Associated Press quoted Aref Ali Nayed, a member of the rebel-led government’s stabilization team, as saying Friday.

Few activists or diplomats expect a rapid end to the situation in Syria. Reports of army defections surfaced in recent days, but because the Syrian government has not allowed journalists into the country, they are impossible to confirm. Earlier this week, the attorney general of Hama, a restive central city, appeared in a video saying he was resigning from his position in protest against the crackdown. The Syrian state news agency said that the official, Adnan Bakkour, had been kidnapped and forced to make the recording, an allegation he he denied in a second video.

Most of the deaths Friday were in the suburbs of the capital, Damascus, according to the Local Coordination Committees, a network of activist groups, and the London-based Syrian Observatory for Human Rights. Other deaths were reported near the central city of Homs and in Deir al-Zour, in the east. An activist working with the Red Crescent in Homs said that a hospital there was having trouble treating wounded on Thursday night because the government-controlled blood bank would not give it blood.