The European Union last year awarded Iranian lawyer Nasrin Sotoudeh and film director Jafar Panahi the 2012 Sakharov Prize for Freedom of Thought. Sotoudeh was freed briefly but was returned to prison in late January. Panahi remains jailed.
Human Rights Watch said crackdowns like those show the government in Tehran is keen to silence critics.
"The Iranian authorities' obsessive clampdown on rights defenders, journalists and the Internet suggests they are intent on clearing the field of all opposition for the upcoming presidential election," Sarah Leah Whitson, director of Middle East Programs at the rights group, said in a statement from New York.
Iran is to have presidential elections in June. Incumbent President Mahmoud Ahmadinejad can't run because of term limits. Opposition leaders Mir Hossein Mousavi, Mehdi Karroubi and other reformists, meanwhile, are under house arrest and ruled ineligible to take part in politics.
"The lesson that free and fair elections, not increasing repression, will lead to legitimacy and long-term stability seems to be lost on them," said Whitson.
Mousavi was considered a favorite in 2009 elections. Ahmadinejad's unexpected victory caused unrest not seen in Iran since the country's Islamic revolution in 1979.