Campaigning officially started on Friday for Iran's May presidential election, pitting pragmatist President Hassan Rouhani against hardliners just as the United States reassesses its policy on the Islamic Republic.

A hardline watchdog body in charge of vetting candidates and laws, the Guardian Council, approved six candidates on Thursday for the May 19 vote - including Rouhani - but hardline former President Mahmoud Ahmadinejad was disqualified.

A witness who was near Ahmadinejad's house in eastern Tehran on Thursday night told Reuters that "around 50 police officers had blocked two ends of the street to his house to prevent possible gathering of his supporters".

Iranian police fanned out across Tehran's main squares overnight after the names of the candidates were announced, according to videos posted on social media.

Ahmadinejad, an adversary of the West during his time on power, surprised Iran's clerical establishment by registering as a candidate, defying Iran's top authority Supreme Leader Ayatollah Ali Khamenei's warning not to enter the race.

His re-election in 2009 ignited an eight-month firestorm of street protests. His pro-reform rivals said the vote was rigged.

Supporters of the six successful candidates had started campaigning on social media last week. Iran blocks access to Facebook, Twitter and YouTube but millions of Iranians use virtual private networks (VPNs) to access those sites.

Iran's top leaders regard the election in part as a show of defiance against renewed U.S. pressure under President Donald Trump, and have called for a high turnout to strengthen the clerical establishment's legitimacy.

"The election is a very difficult and important test for all of us ... a high turnout will show to the world that the establishment enjoys the strong backing and support of its people," Tehran Friday prayer leader Ayatollah Mohammadali Movahedi-Kermani told worshippers, state TV reported.

Rouhani won a landslide victory in 2013 on a platform of ending Iran's diplomatic isolation and reviving the country's crippling sanction-hit economy.

On April 4, the U.S. Senate has delayed a bill to slap new sanctions on Iran due to concerns about the election.

On Wednesday, U.S. Secretary of State Rex Tillerson accused Iran of "alarming ongoing provocations" to destabilise countries in the Middle East.

"A comprehensive Iran policy requires we address all of the threats posed by Iran and it is clear there are many," he said.

Rouhani engineered Iran's 2015 nuclear deal with world powers that secured removal of most of international sanctions against Tehran. But hardliners say he has failed to boost the economy despite lifting of sanctions last year.

Analysts say the biggest challenge to Rouhani - himself endorsed by moderates and prominent conservatives, including parliament speaker Ali Larijani - is influential mid-ranking cleric Ebrahim Raisi, who is close to Khamenei.

The four other candidates are Iran's first Vice President Eshaq Jahangiri, former conservative culture minister Mostafa Mirsalim, former pro-reform vice president Mostafa Hashemitaba and Tehran Mayor Mohammad Bagher Qalibaf.