The RUC is believed to be preparing contingency plans to deal with any major public disorder arising at Apprentice Boys' parades tomorrow in Derry and Belfast's lower Ormeau Road.
Nationalists are considering blocking the route of the parades and a sit-down protest by residents could lead to confrontation with the RUC.
The Parades Commission chairman, Mr Alistair Graham, has appealed to all sides to act within the law.
Mr Graham yesterday had an hour-long meeting with Sinn Fein. Afterwards he said he had heard nothing that would cause him to reverse his decision to allow the marches.
The Government has said it recognises the "huge frustration on the ground" in nationalist communities over the decision to approve the Apprentice Boys march along the Derry city walls.
It is also understood that a senior Irish civil servant from the Northern Ireland Secretariat has been called back from holiday to conduct, what a spokesman described as, "a full review of the present situation in relation to the Apprentice Boys' parade".
The official is expected to report to the Government on the mood in nationalist areas in the North.
A spokesman said the Government would have preferred "not to have seen the Commission coming down on one side or the other".
He said there was regret in Dublin that dialogue had not led to an agreement that was "sensitive to the needs of the nationalist community". He emphasised that the Government was not in conflict with the work of the Commission, but said there was "anxiety at the present situation".
Mr Gerard Rice, of the Lower Ormeau Concerned Community, said an attempt to block the Ormeau Road was "a very definite possibility".
"This community very firmly believes that it must show opposition to the parade and we have no choice but to oppose the parade on Saturday morning," Mr Rice said.
A Bogside Residents Group spokesman confirmed it would hold a rally in the Diamond at the same time as the Apprentice Boys' parade. "Ten thousand loyalists will be laying siege to our city centre for six hours in what is nothing more than a coat-trailing exercise," he said.
And while the spokesman refused to comment on the exact nature of the protest, sources close to the group said they would do all they could to block the route and might stage a sit-down protest.
The Apprentice Boys governor, Mr Alistair Simpson, condemned the "veiled threat of violence" from the BRG which he accused of attempting "to hold the Apprentice Boys and this city to ransom". He urged his members to "walk with dignity, pride and respect".
The Rev Ian Paisley accused nationalist residents groups of "bigotry and hatred" but said the Apprentice Boys must not be goaded.
Ulster Unionist Assembly member, Mr Peter Weir, also said nationalist groups had shown their "true bigoted colours".
However, Sinn Fein called on the Apprentice Boys to reconsider marching down the Lower Ormeau Road and said the organisation would gain the moral high ground if it cancelled its plans.
The people of Derry would be prepared to accommodate the Apprentice Boys in the city if the Apprentice Boys accommodated the wishes of the residents of the Lower Ormeau, a party spokesman said.
A holdall containing a homemade bomb, a shotgun and ammunition was found yesterday at a west Belfast primary school, police said.
Workmen at St John the Baptist school in Finaghy Road North made the discovery in a boiler house at about 12.15 p.m.