Hundreds of protesters joined a demonstration outside the Iranian embassy in London on Saturday, as several hundred more gathered for similar rallies across Europe in cities including Amsterdam, Paris, Prague and Geneva.
Hours earlier, demonstrations took place in South Korea, Japan and Australia.
In Canberra, the Australian capital, some protesters called on Western governments to take a stronger stand against the Iranian government.
"The Australian government should reject the Iranian ambassador - send a big signal," Ardeshir Gholipour, a former Iranian political prisoner, told The Associated Press news agency at the demonstration in Canberra.
Iranian authorities cracked down on opposition supporters after Mahmoud Ahmadinejad's controversial re-election as president, after a vote on June 12, triggered violent protests across the country.
'Abuse of power'
Dr Aaron Rhodes, a member of United for Iran, the coalition organising the global "day of action", said hundreds of opposition supporters were still imprisoned without due process, more than a month after the election.
He told Al Jazeera that a lack of transparency has deprived those detained of their proper rights.
"Our message is very simple," Rhodes said.
"We're supporting civil and human rights in Iran and we're calling upon the government in Iran to cease their abuse of power, cease the imprisonment of innocent people and the torture of detainees and stop the violence against people who are simply trying to exercise their internationally protected human right to peacefully protest."
Although the protests have largely been quelled, political tensions continue to run high in Iran with the defeated candidates refusing to accept the result.
Mir Hossein Mousavi and Mehdi Karroubi, two defeated presidential candidates, say the vote was rigged.
On Saturday, the two men, along with Mohammad Khatami, an ex-president, jointly called on Iran's religious leaders to stop the "oppression" of their supporters.
In a statement posted on Mousavi's website, they said: "We want you, the top clerics, to remind the authorities of the harmful outcome of not abiding by the law and prevent them from spreading oppression in the Islamic republic."
In a separate statement, Karroubi accused Iranian intelligence agents of handling opposition supporters with harsher methods than Israelis in the occupied Palestinian territories, the Reuters news agency reported, citing his website.
"The behaviour of Iran's security agents is worse than those of the Zionist [Israel] in the occupied Palestine," he said in a letter addressed to Gholamhossein Mohseni-Ejei, Iran's intelligence minister.
"Can you put detainees under mental pressure in mosques, schools and in the basements of government offices?"
But Kian Mokhtari, an Iranian political commentator, accused leaders like Mousavi of rabble rousing.
"If there is a problem with the elections he should come forward instead of rabble rousing. He is putting Iranian youth in harms way," Mokhtari told Al Jazeera.
"Damage to public property is a crime. [Those responsible] will receive sentences of three to six months and a fine. It is the same in Europe.
"There are no political prisoners in Iran," he said.
"A few hundred thousand people overseas cannot impose their will on 70 million people in Iran."