Groups including Human Rights Watch and Amnesty International are backing a global day of action, with protests planned in more than 80 cities.
The protesters want Iranian authorities to release what they say are hundreds, or even thousands, of people detained during protests that followed the presidential election last month that returned Mahmoud Ahmadinejad to power.
Protesters were gathering outside the Iranian embassy in London, one of a series of events in cities across Europe. In Brussels, Belgium, protesters held placards carrying images of the detained or dead, including Neda Agha Soltan, the 27-year-old whose death
- beamed around the world on the Internet - became a rallying cry for opponents of the regime.
In Australia, there were small protests in Sydney, Melbourne, Brisbane, Adelaide and the capital, Canberra.
About 50 people waved flags and banners reading "Stop torture" and "Iran election was a fraud" on the steps of the parliament of Victoria state in Melbourne.
In Canberra, Ardeshir Gholipour, a 41-year-old refugee from Iran who arrived in Australia in 2000 after spending two years in an Iranian prison for political protests, said Western governments should take a tougher stand against the Ahmadinejad regime.
"The Australian government should reject the Iranian ambassador - send a big signal," he said.
About 80 people gathered in Tokyo, draping green scarves - the color of Iran's protest movement
- around their necks and lighting candles.
The crowd watched recent news footage of Iran and a slideshow on a screen set up in Miyashita Park in downtown Tokyo to the backdrop of rock music, and applauded speeches calling for "a free Iran" and fair elections.
