Members of a moderate Iranian presidential candidate's campaign team were arrested on Saturday, an opposition website said, after a rally in which he criticised heightened security measures in the country.
Supporters of Hassan Rohani also shouted slogans in support of detained opposition leader Mirhossein Mousavi at the rally in Tehran, according to a video of the event posted on YouTube.
Rohani's criticism could be seen as potentially inflammatory before the June 14 election which authorities hope to tightly control, eager to prevent a repeat of the mass protests after the 2009 poll.
Those protests, by the opposition "Green Movement" against what they saw as a fraudulent win for conservative President Mahmoud Ahmadinejad, were crushed by security forces.
Mousavi, his wife, and fellow reformist Mehdi Karoubi have been under house arrest for over two years. Many activists have been jailed or have left Iran.
This year's list of eight presidential candidates, vetted by Iran's electoral watchdog, is dominated by hardline and conservative candidates close to supreme leader Ayatollah Ali Khamenei.
Cleric Rohani - the most prominent moderate still in the race - addressed a boisterous crowd of hundreds of young supporters in Tehran's northern neighbourhood of Jamaran on Saturday, the video showed.
"Why should there be a security atmosphere everywhere?" he said. "In the streets, universities, schools, organisations, we must put an end to this security atmosphere."
Rohani encouraged his supporters to head to the ballot box in droves. "Our people deserve more peace, more freedom, more prosperity, more honour and more security. This is only possible with your presence. Don't let them discourage you," he said.
"If people don't show up to vote on election day, they have effectively left the field to their opponent."
The crowd punctuated his speech with chants such as "Rohani, Rohani, we support you." Supporters also shouted: "Political prisoners must be released" and "Praise to Mousavi, welcome to Rohani", according to the video, which was publicised on a Twitter account supporting Rohani's campaign.
Following the rally, plainclothes security officers and police units blocked nearby streets, reported Sahamnews, an opposition website close to Karoubi. Several of Rohani's campaign workers were arrested including Saeed Allah-Bedashti, who heads the youth division, it said.
Rohani, Iran's former nuclear negotiator, has been criticised by the campaign of current negotiator Saeed Jalili for being too compromising with world powers in talks over Iran's disputed atomic programme. Rohani has rejected that charge.