Iran was braced for a fifth day of unrest today as the government intensified its crackdown on opposition figures with the arrest of dozens of leading critics and issued a further warning against reporting of the protest movement.

Saeed Laylaz, a leading journalist and a critic of government policy often quoted by foreign media, was among the latest to be detained, as protesters prepared for more demonstrations in Tehran.

"Iranian intelligence and security forces are using the public protests to engage in what appears to be a major purge of reform-oriented individuals whose situations in detention could be life-threatening," said Aaron Rhodes, a spokesman for the International Campaign for Human Rights in Iran.

There is also concern for the health of Saeed Hajarian, a former adviser to Mohammad Khatami and supporter of the opposition candidate Mir Hossein Mousavi, who was arrested yesterday. Hajarian, who was badly impaired in both mobility and speech in an assassination attempt nine years ago, needs constant medical attention and is unlikely to get it in the current circumstances.

Human rights groups said at least 100 people had been arrested in the city of Tabriz, a historic centre of protest and a Mousavi stronghold.

In a fresh challenge to the government, Mousavi urged supporters to stage peaceful protests or gather in mosques tomorrow to mourn those killed in the protests.

"A number of our countrymen were wounded or martyred," Mousavi said in a statement on his website.

"I ask the people to express their solidarity with the families … by coming together in mosques or taking part in peaceful demonstrations."

Mousavi added that he would also take part in the day of mourning.

He also repeated his call for "a new presidential election that will not repeat the shameful fraud from the previous election".

In a sign of the divisions in Iran's leadership, the interior ministry ordered an investigation into an attack on university students which they say was carried out by militia and police. It came a day after Iran's influential speaker of parliament, Ali Larijani, condemned the assault on the dormitory of Tehran University.

There were, however, further signs of a crackdown today as the powerful Revolutionary Guards warned Iranian websites and blogs to remove content that might "create tension" or face legal action, according to the Associated Press.

Much information about the protests has come from blogs and websites such as YouTube, Facebook and Twitter. The government yesterday barred foreign media from leaving their offices to report on the demonstrations, and reporters' visas have not been renewed.

Amid fears that tension might lead to further bloodshed, Gordon Brown today urged Iran to listen to its people. "The elections are a matter for the Iranian people, but if there are serious questions that are now being asked about the conduct of the elections, they have got to be answered," he said. "There must be no violence in response to peaceful protests."

Iran's foreign ministry summoned the British ambassador and his European counterparts to criticise their "interventionist and impudent" responses to the disputed election.

Meanwhile, the prosecutor general of the central province of Isfahan warned that those behind post-election unrest could face the death penalty under Islamic law. Mohammadreza Habibi alleged that anti-government protesters were being controlled from outside Iran and urged them to stop what he described as "criminal activities", the Fars news agency reported.

There were also unconfirmed reports that Mohammad Asgari, who was responsible for the security of the IT network in Iran's interior ministry, was killed yesterday in a suspicious car accident in Tehran. Asgari had reportedly leaked evidence that the elections were rigged to alter the votes from the provinces. Asgari was said to have leaked information that showed Mousavi had won almost 19m votes, and should therefore be president.

Iran's supreme leader, Ayatollah Ali Khamenei, last night appeared to rule out any change to the outcome of last week's disputed poll by referring to Mahmoud Ahmadinejad as the "elected president".

Khamenei last night dismissed the protests as the work of "tension seekers". Khamenei's appeal for calm after four days of protests in Tehran followed an apparent concession when the regime promised to recount some votes cast in Friday's disputed election, officially won by Ahmadinejad. But the authorities rejected demands by the defeated Mousavi to annul the election.