Intense negotiations between the P5+1 and Iran have ended with an agreement, under which Tehran will reduce its uranium enrichment capacity in exchange for phased sanctions relief.
US President Barack Obama said a "historic understanding" had been reached with Iran.
The P5+1, the US, UK, France, China and Russia plus Germany - and Iran, have been in negotiations for months and a deadline to reach an agreement on March 31 was missed.
Under the deal, Iran will face more inspections than any other country, President Obama said.
"If Iran cheats, the world will know it," he said, adding that the deal was based not on trust but on "unprecedented verification".
According to a US factsheet, the outline deal includes the following conditions:
- Iran will reduce its installed centrifuges - used to enrich uranium - by two-thirds and reduce its stockpile of low-enriched uranium
- The centrifuges that are no longer in use will be placed in storage, monitored by the International Atomic Energy Agency (IAEA)
- All of Iran's nuclear facilities will be subject to regular IAEA inspections
- Iran will redesign its heavy-water reactor in Arak so that it cannot produce weapons-grade plutonium
US and EU sanctions related to Iran's nuclear programme will be lifted in phases, but can be brought back if Iran does not meet its obligations.
The deal was welcomed by Iran, Russia and the EU, but faced criticism from Israel, who warned that it could threaten its survival.