BBC correspondent Tom Coulter: this does not augur well for the rest of the marching season (25")

Those treated in hospital included a policewoman injured when a blast bomb was thrown at RUC lines.

The government said the disturbances, on the well-known flashpoint area of Garvaghy Road in Portadown, County Armagh, had been orchestrated.

The Northern Ireland Security Minister Adam Ingram said: "The actions of those who protested were deliberately destructive and unacceptable."

The police take up positions

He said the people of Northern Ireland wanted peace and had reiterated this in last week's referendum.

"They do not want their towns and villages blighted by trouble of this sort and attacks on security forces will not help solve questions about parades.

"In the spirit of recent political developments I hope we can look forward to people resolving their differences through discussion and appreciation of each other's views - not through violence."

The marchers enter the Garvaghy Road

The violence erupted as nationalists objected to a march by children from the Protestant junior Orange Order along a stretch of the Garvaghy Road.

Several hundred Catholic residents gathered to protest and were confronted by angry Loyalists.

Police in riot gear kept the two groups apart but as tension rose, the police came under attack.

At the height of the trouble, blast, petrol and paint bombs as well as stones and bottles were thrown at police and troops.

Breandan Maccionnaith: "No equality"

Officers fired salvos of baton rounds but it was several hours before the crowds dispersed.

Up to 400 nationalists were involved in the disturbances - the worst in Northern Ireland since the signing of the Good Friday Agreement on April 10. Loyalists were also involved in attacks on police.

Bertie Ahern: danger from unrest at marches

The violence comes days after the Irish Prime Minister, Bertie Ahern, met residents of Garvaghy Road and warned of the danger that unrest at marches posed to peace.

A residents' spokesman said hopes that the Stormont agreement would bring change had been dashed. Breandan Maccionnaith blamed the RUC for not treating Catholics fairly.

Troops were called into Garvaghy Road last July

The Garvaghy Road, a Catholic zone of Portadown - a largely Protestant town - has been the scene of serious rioting in recent years.

There were clashes last July when security forces were sent into the area to allow the Orange Order to march through it.

Days later, with the province on the brink of turmoil, the Orange Order called off or re-routed several contentious parades.

Clearing up on Sunday

The trouble on Saturday erupted as leaders of the Orange Order faced new demands by Garvaghy Road residents to meet them to discuss plans for the Drumcree march on July 5.

The summer marching season, when Protestants and Catholic nationalists hold parades to mark previous victories over each other, is traditionally a time of tension in Northern Ireland.

Meanwhile, police investigating the murder of a man in his early 20s who was shot dead say the motive was not sectarian.