Mathew and Ben Carlin enjoyed the parades in Belfast

The crowd gave the largest cheer to pupils from Holy Cross Girls' Primary School and their parents who led the north Belfast parade.

Their school was the scene of months of loyalist protests last year, following sectarian tensions in the Ardoyne area.

Organiser of the Belfast parade Irene Sherry said there had been a good turnout despite the weather.

"Everything has gone well. There are thousands here. We have a great line-up of entertainment and hopefully everybody will have a brilliant day," she said.

In Belfast puppeteers teamed DUP leader Ian Paisley with Saddam Hussein

Before the parade, three security alerts in west Belfast were declared hoaxes after the Army carried out controlled explosions on suspicious objects.

In Derry, singer Brian Kennedy headlined a free outdoor concert in the city's Guildhall Square.

Later an Ulster-Scots concert is to be held at the Ebrington centre in Derry.

Parades were also held in Ballycastle, Dunloy and Ballymoney in County Antrim, Dungiven, Kilrea and Magherfelt in County Derry and Portadown and Lurgan in County Armagh.

On Saturday, there were parades in Warrenpoint, Kilkeel and Newry in County Down.

Dublin celebrations

An estimated half-a-million people turned out to watch the huge parade through Dublin city centre on Sunday.

About 2,000 marchers and bands took part in the 2.5m euro parade from St Patrick's Cathedral into the city centre.

Irish soccer manager Mick McCarthy was the grand marshall of the national parade, which was broadcast live on Irish television and was streamed on the web.

Football manager Mick McCarthey is grand marshall of the Dublin parade

The first day of celebrations climaxed in Dublin on Saturday night with a spectacular firework display in the city centre over the River Liffey.

Throughout Saturday street entertainers and bands performed during a street carnival in the city centre.

The celebrations were only marred by an incident in which a 19-year-old man drowned trying to swim across the Liffey on Saturday night.

On Sunday, celebrations were also held in Galway, Navan, Limerick and Sligo.

In London, the first official celebration of St Patrick's Day was organised by Mayor Ken Livingstone, in partnership with Irish community organisations.

The festival, which began with a parade from Westminster Cathedral to Trafalgar Square, was held with the support of UK Prime Minister Tony Blair and his Irish counterpart Bertie Ahern.

Mr Ahern addressed the Trafalgar Square crowds via video link.

New York parade controversy

Meanwhile, one of biggest St Patrick's Day parades in the United States, in New York state, went ahead despite controversy over the involvement of a convicted IRA bomber.

Brian Pearson was chosen to be grand marshal of the giant parade in Pearl River, 25 miles from New York, which is the second biggest in the state after New York city's celebrations, and one of the largest in the US.

Pearson, 50, was granted political asylum in the United States in 1997 despite the objections of the US government, after spending 12 years in prison for blowing up two police stations in Northern Ireland.

But his presence has prompted an unprecedented boycott of the parade by police and firefighters' organisations who do not want to be associated with terrorism in the wake of the September 11 attack.

On Saturday, hundreds of thousands of people turned out for the national parade and a two minute silence was held to remember the victims of the World Trade Center terrorist attacks on 11 September.

Many of the emergency workers who died came from Irish-American communities.