The PSNI is to march along with dissident republicans in a St Patrick's Day parade in New York at which hundreds of anti-British banners will be on show.
It's the first time the force has taken part in the parade – the world's largest. Its presence has sparked huge controversy.
Hardline republicans warn that the police will be booed and jeered by protesters on the sidewalks of the American city today.
The PSNI's participation in the parade hung in the balance all weekend. Following a deluge of protests from militant Irish Americans, organisers rescinded their original invitation to the PSNI.
It was reinstated only after frantic behind-the-scenes intervention from Sinn Fein and the Irish Government, who said banning the PSNI would send out "a disastrous message".
Six officers will march beside gardai in a display of "mutual respect" designed to show Irish America the political progress made in Northern Ireland.
But strong opponents of the Belfast Agreement are also taking part in the parade, including former Noraid publicity director, New York lawyer Martin Galvin.
In 1984 Galvin – who was banned from entering the UK – defied the authorities by appearing at an anti-internment rally in west Belfast. In an attempt to arrest him the RUC stormed the crowd and shot dead Sean Downes with a plastic bullet.
Mr Galvin said it was bizarre that members of "a British police force" will be in a parade at which the official banner 'England Out Of Ireland' will be carried by hundreds of marchers.
"There will be no physical threats to them today but you will hear 'SS RUC' and other chants from the crowd to show this police force they aren't wanted here because they uphold British rule in Ireland," he said.
The PSNI's participation in the march is supported by Sinn Fein and the Irish Government, who are confident that protest will be restricted to "a small hardcore".
The protest comes three days after police in west Belfast came under attack from suspected dissident republican terrorists. Four officers escaped injury when their patrol was targeted with a remote control bomb on the Falls Road.
Today, some 200,000 marchers will walk up Fifth Avenue in front of a million-strong crowd. The vast majority have no interest in politics, and many aren't Irish.
The PSNI marchers will be led by a chief inspector. When their participation was announced, militant Irish Americans bombarded the organisers with phone calls.
On Friday the St Patrick's Day parade organisers issued a statement which said: "While the decision to invite the PSNI was made in an effort to foster peace, we must stand behind those who help make our parade the greatest in the world. Therefore, we have rescinded the invitation and the PSNI will not march."
Sinn Fein and the Irish Government immediately instigated behind-the-scenes talks with organisers.
Gerry Adams said withdrawing the invite was "a mistake which needs rectified".
Martin McGuinness said the PSNI marching with gardai "will be welcomed by all who support peace in Ireland".
The organisers then reversed the decision to ban the PSNI.
A PSNI spokesman said he was aware of concerns surround the force's participation but officers were definitely taking part.
The parade has long been associated with republican sentiment. In the 1980s it chose as chief marcher Joe Doherty, a Belfast IRA man who could not attend as he was in a New York jail awaiting extradition to the UK.
The parade remains controversial because lesbian and gay groups are banned.
Dissident republicans are not alone in opposing PSNI participation in the parade.
New York Mayor Bill de Blasio is boycotting today's march because of the ban which gay rights activists say violates human rights.
LGBT groups have been banned for decades due to opposition of Catholic Irish Americans.
Gay rights campaigners last week urged the force to reconsider its decision to take part because of the gay rights controversy.
Veteran gay rights activist Jeff Dudgeon said PSNI participation "sends a discordant symbol to the gay community in Belfast who have stood in solidarity for many years with our brothers and sisters in New York over their exclusion".
He told The Guardian: "It is rather disheartening given the fact that de Blasio has decided not to attend, but the PSNI's need for acceptance in the States would always have taken precedence.
"In truth, the police's commitment to effective responses to LGBT needs in Northern Ireland will be the measure that matters, and the jury is still out in that department."
The New York City St Patrick's Day parade is the biggest in the world and goes through Manhattan. It began on March 17, 1762. It has been held for the past 252 years and honours Ireland's patron saint and the Archdiocese of New York. The parade is reviewed from the steps of Saint Patrick's Cathedral on Fifth Avenue by the Archbishop of New York. Today, 200,000 will walk up Fifth Avenue in front of a million spectators.
TIMELINE OF CONTROVERSY:
New York Mayor Bill de Blasio says he will not be taking part in his city’s St Patrick’s Day parade in solidarity with the gay community. Lesbian and gay groups are banned from taking part because of opposition of conservative Catholics with an Irish Americans background. He is the first mayor to boycott the parade.
It is announced the PSNI is to march in New York's parade for the first time. Organisers say six PSNI officers will march alongside members of An Garda Siochana. They were chosen because of their participation in the World Police and Fire Games in Belfast.
Parade organisers issue a statement which says: “While the decision to invite the PSNI was made in an effort to foster peace, we must stand behind those who help make our parade the greatest in the world. Therefore, we have rescinded the invitation and the PSNI will not march in the parade.”
The PSNI says its officers are travelling to New York as it has received no notification the invitation has been rescinded. The statement is removed from Facebook. Sinn Fein and the Irish government instigate discussions. Gerry Adams said withdrawing the invitation was “a mistake which needs rectified”.
Organisers confirm the PSNI will take part in the march after a U-turn on the ban.
The St Patrick’s Day Parade will be held in Manhattan with the PSNI taking part for the first time alongside gardai. Dissident republican groups will also be taking part.