Although the republican street violence which followed last Sunday's Orange march at Drumcree had largely subsided yesterday, apprehension remains at a high level in advance of Saturday's 12 July marches, the height of the marching season.
The two possible flashpoints will be in Londonderry, where 10,000 Orangemen are due to assemble in the face of already vocal opposition from local Catholic residents, and on the Ormeau Road in south Belfast.
The fact that major demonstrations are to take place at more than a dozen venues all over Northern Ireland means that Army and RUC sources will be stretched. The 400 extra troops, from the first battalion, the Stafford Regiment, will arrive today. Their deployment was ordered by ministers following advice from RUC Chief Constable, Ronnie Flanagan, and Army General Officer Commanding, Lieutenant General Sir Rupert Smith.
Last night, John Hume, the nationalist Social and Democratic Labour Party leader, urged Tony Blair in private talks at Westminster to impose a moratorium on more marches in Ulster next weekend in an attempt to stem the wave of rioting, shooting and firebombings.
"We expressed to him very strongly our feeling about the situation in the Garvaghy Road and the deep anger that exists in the community," Mr Hume said after the meeting.
However, in spite of the clear anger among SDLP MPs at the decision to drive ahead with the Orange march in Portadown, Mr Hume signalled a conciliatory move towards the beleaguered Secretary of State for Northern Ireland, Mo Mowlam.
He refused to criticise Ms Mowlam over the leaked document which showed that civil servants had been planning for the march three weeks before it took place, and he said she had explained at the meeting that it was one of a series of options they had considered.
The SDLP leader and his deputy, Seamas Mallon, were not told about the deployment of more troops until after the meeting with Mr Blair, but they said it showed the need to reduce the tension on the streets in the province. Mr Blair was said by Mr Hume to be considering his call for a ban on some specified marches which they fear could lead to violence.
The Prime Minister gave his strongest support so far to his Northern Ireland secretary in the Commons when he called for calm on all sides.
Mr Blair said: "The situation in Northern Ireland over the past few days has been appalling and it is tragic for all the people in Northern Ireland.
"We have been trying - and in particular Ms Mowlam has been trying, to whom I pay tribute to her courage and determination - to do the best in good faith in a situation in which all the options available are difficult and hard."
He called on all sides "to try as best they can, recognising all the pressures that are on them, just as there are pressures on us, to keep the wider process for a lasting political settlement going".
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