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World leaders reached a framework on Thursday for a historic deal meant to block Iran from developing a nuclear weapon.

Iran, which insists that its nuclear ambitions are peaceful, agreed to limits on its program to enrich uranium, and it agreed not to make weapons-grade plutonium — two means of building a bomb.

Iran also agreed to international inspections that President Barack Obama said would be the toughest on any country in the world. If Iran makes good on its pledges, world powers agreed that they would lift punishing economic sanctions.

Obama called it a good deal that would make the world safer and “cut off every pathway” to an Iranian nuclear weapon. He also said that it was vastly better than bombing Iranian nuclear facilities and starting another war in the Middle East.

“If Iran cheats, the world will know it,” he said from the White House Rose Garden. “If we see something suspicious, we will inspect it.”

Read details of the framework

Iran and six world powers, including the United States, had been in negotiations in Switzerland since March 26 on the nuclear program. They agreed to work toward a final text by June 30.

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Federica Mogherini, the European Union foreign policy chief, called it a “decisive step.” In Iran, people celebrated in the streets.

Javad Zarif, the Iranian foreign minister, told reporters that the agreement demonstrates that “we can in fact solve problems, open new horizons and move forward.”

The deal would cut Iran’s stockpiles of enriched uranium by 98 percent for 15 years, and it would cut Iran’s installed centrifuges by two-thirds for 10 years, Secretary of State John Kerry.

He also said that it would increase Iran’s hypothetical “breakout time” — how long it would take Iran to speed up enrichment and produce enough fissile material for a nuclear bomb — to one year, from the current two to three months.

The countries had set a deadline of Tuesday for a framework, but they blew past it and kept talking. They worked through the night into Thursday, taking a break of just a few hours for sleep.

Republicans in Congress have strongly opposed a deal with Iran. Obama warned that if Congress kills it, “it’s the United States that will be blamed for the failure of diplomacy.”

“The issues at stake here are bigger than politics,” the president said. “These are matters of war and peace, and they should be evaluated based on the facts.”

He drew parallels to arms-control agreements that Presidents Richard Nixon and Ronald Reagan reached with the Soviet Union, “a far more dangerous adversary.”

France warned that there was still work to do.

“France will be watchful, as it always is in step with its partners, to ensure that a credible, verifiable agreement be established under which the international community can be sure Iran will not be in a position to have access to nuclear arms,” President Francois Hollande said.

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Big day: #EU, P5+1, and #Iran now have parameters to resolve major issues on nuclear program. Back to work soon on a final deal. — John Kerry (@JohnKerry) April 2, 2015

Found solutions. Ready to start drafting immediately. — Javad Zarif (@JZarif) April 2, 2015

— Erin McClam