Security forces and demonstrators clash in Cairo after former interior minister's trial is put off without explanation

Clashes between protesters and security forces engulfed Cairo once again on Tuesday night, as the fiercest street battles since the fall of Hosni Mubarak left dozens injured.

Fighting began after dark, following earlier protests by relatives of those killed during this spring's uprising.

Armed central security police showered Tahrir Square with tear gas canisters and fired bullets into the air as several thousand demonstrators amassed and called for the resignation of Egypt's de facto head of state, Field Marshall Mohamed Hussein Tantawi.

Some members of the crowd tore up paving stones and threw them towards police lines.

The Guardian witnessed successive volleys of tear gas launched into the square and surrounding streets by government forces, including towards areas where ambulances had congregated to treat the wounded. Injured protesters, mostly with head wounds and gas inhalation, were carried to safety on the shoulders of fellow demonstrators.

"Mubarak was nothing – this is the revolution," said one man caught by tear gas.

The interior ministry blamed a group of "thugs" for the unrest, claiming that they had disrupted an event held earlier in the day to commemorate the martyrs of the revolution and went on to attack the ministry headquarters.

Protesters vociferously denied that suggestion, insisting that the police had attacked unarmed relatives of the martyrs – an account seemingly backed up by unverified videos posted on YouTube.

The violence came after the trial of the former interior minister, Habib al-Adly, on the charge of unlawfully killing pro-change protesters had been delayed by a judge this week, with no reason given to the public. "People are saying that we've replaced one Habib al-Adly with another," said Mostafa Hussein, a 30-year-old activist in Tahrir.

"They believe the interior ministry has returned to its former incarnation under the Mubarak regime."

Also on Tuesday, an administrative court ordered the dissolution of Egypt's 1750 municipal councils, a form of local government that was almost entirely controlled by the old ruling NDP party and was a key tool of control for Mubarak's dictatorship.

Revolutionary activists have long demanded that the councils be disbanded, pointing out that in "elections" held in 2008 over 99% of open seats were awarded to Mubarak allies. However it is not yet clear whether the interim government will implement the judicial ruling.

Almost 1,000 people are believed to have died in the year's 25 January revolution, in which Mubarak's black-suited central security troops were beaten off the streets by a mass uprising. Since the resignation of the former president, Egypt has been run by the armed forces who have promised a swift transition to a democratic civilian government. But many fear that the pace of change has been too slow, accusing generals of hijacking the revolution and failing to hold members of the old regime to account.

"For those that had loved ones killed in January – and indeed all those who lived through that time – just the sight of the Amin al-Markazi [central security forces] on the streets is a provocation in itself," a doctor who had been treating the injured told the Guardian.

"People are shouting 'our revolution is being stolen'. The situation is very tense."

In the early hours of the morning, Tahrir Square remained shrouded in gas with the battle continuing. Activists took to Twitter and other social media sites to call for medical supplies and people to join the gathering.