Eritrean army tanks besieged the information ministry in central Asmara yesterday after some 200 mutineers seized the building to call for political reform, diplomatic sources said yesterday.
No shots had been fired and the rest of the city appeared calm, the diplomats added, although very few details were available.
"The ministry of information is under siege," a diplomat said.
Amanuel Ghirmai, an Eritrean journalist in Paris for independent Radio Erena, said that around 100 army mutineers stormed the hilltop ministry - towering over the capital of the Red Sea state - early yesterday.
They reportedly ordered news readers at the government-run television and radio station - the only source of media for the authoritarian state - to read a statement that they will implement the country's constitution.
The statement also reportedly ordered the release of prisoners of conscience.
"We do not know who is leading the situation ... everybody has been put into the same room [in the ministry]," Amanuel said, citing sources in Asmara.
Britain's foreign office updated its travel advice to say it had received reports of "unusual military movements in and around Asmara". The reports were not possible to confirm independently, and all calls to government officials were unanswered.
Impoverished Eritrea beats even North Korea to rank last out of 179 countries on the Press Freedom Index of the Paris-based media watchdog Reporters Without Borders.
All independent media was shut down after President Issaias Afeworki launched a draconian purge in 2001, while the Horn of Africa nation expelled the last registered foreign correspondent in 2010.
However, Eritrean expatriates who had spoken to compatriots in Asmara said they could confirm reports that troops had been deployed at the information ministry, and that state television Eri-TV had stopped broadcasting.
All Eritrea's public media are recorded and broadcast from the ministry. "The local transmission has been cut, the only satellite signal is airing some archives," Amanuel added.
Issaias has ruled Eritrea with an iron grip from independence in 1993, following an epic 30-year liberation war with neighbouring Ethiopia.