Several hundred republicans from across the North are set to hold a picket at Maghaberry prison today in protest at what they say is a "powder keg" waiting to explode in the jail.

They claim that 33 republican dissident prisoners are being locked in their cells for 21 hours a day, are often not receiving their evening meal, and are searched sometimes every four feet as they walk the prison corridor.

In a rare show of unity from the various dissident republican groups, members of the 32 County Sovereignty Movement, Eirigí, Republican Network for Unity, the IRSP and unaligned republicans will join forces for today's protest as tensions mount in the jail.

One organiser, Carl Reilly (35) from west Belfast, said: "Maghaberry is a powder keg. It's been building for a very long time over the denial of basic rights. These men are locked in their cells for 20 hours one day, 21 hours the next.

"Before, three men were allowed out of their cells onto the wing at the same time. Now it's only one – which leads to massive delays in getting a shower and getting food. The men aren't allowed to eat in the canteen. They're let out of their cells, one at a time, to pick up their meal which they're forced to eat alone in their cells.

"When a man is allowed out of his cell, he can be subject to a search every four feet he walks. It's pure harassment."

Reilly, who was released from Maghaberry in 2004 after serving a six-year sentence for dissident activity, said the situation had deteriorated substantially since he was in jail. He said prisoners had been banned from wearing Easter lilies outside their cells.

"That was ludicrous since only republican prisoners are on the wing and every year prison officers wear poppies as big as they can buy."

He blamed the prison service, not the Northern Ireland Office, for the situation.

"The police, the judiciary, and other arms of the state have been subject to some reform but the prison service remains untouched."