BELFAST, Northern Ireland — Secretary of State Hillary Rodham Clinton flew to Belfast on Friday for a brief visit that contrasted calls for peace with the discovery of two bombs unconnected with her diplomacy, underscoring the continued power of sectarian passions almost 15 years after a formal peace accord brokered partly by her husband.

The discovery of the devices elsewhere in the country came after days of chaos on Belfast’s streets when unionists, who seek continued ties with Britain, blocked traffic to protest a decision by the City Council to limit the number of days when Britain’s Union Jack flag is flown at City Hall.

“The violence is a reminder that, although much progress has been made, the hard work of reconciliation and fostering mutual understanding must continue,” Clinton said after meeting some of Northern Ireland’s political leaders.

“The only path forward is a peaceful, democratic one that recognizes the right of others to express their opinions but not to resort to violence,” she said. “There can be no place in the new Northern Ireland for any violence. Any of the remnants of the past need to be quickly and unequivocally condemned.

“The United States will continue to strongly support all those who stand on the side of peace and reconciliation.”

Hours before Clinton arrived from Dublin, four men were arrested late Thursday when a homemade explosive device was found in the city of Londonderry, police officials said. The bomb was discovered in a car rammed by officers investigating the activities of splinter groups that have broken away from the mainstream republican movement opposing British sovereignty in Northern Ireland.

The explosive device was described by the police as “viable.” Army experts defused the bomb after nearby homes were evacuated.

The police also reported Friday that a letter bomb had been discovered in another part of Northern Ireland after a man was observed acting suspiciously near a mailbox.

Officers did not specify the name or address on the letter, but described the bomb as “a viable device capable of causing death or serious injury.”

The tensions are not related to Clinton’s visit, analysts and officials said, but offer a sobering backdrop to what has been depicted as a celebratory visit recalling President Bill Clinton’s diplomatic triumphs in promoting Northern Ireland peace in the 1990s along with the leaders of Britain and Ireland.

On Friday, Hillary Clinton noted that, while “I will soon be a private citizen again” - a reference to her decision to step down as secretary of state - she looked forward to “continuing this work, so the people of this land can prosper in the peace and security that they have worked so hard for, and that they so richly deserve.”

It was Clinton’s second visit as secretary of state since 2009.

In prepared remarks before her departure, she again invoked the peace effort and the challenges to it.

Addressing the people of Northern Ireland, she said: “You are the ones who reminded the world that while a peace deal may be signed at a negotiating table, peace itself takes life at the kitchen table. It must be nurtured in the hearts of people, in the way they live their daily lives and treat their fellow citizens, in the lessons they teach their children.”

Peace, she said, “is always a work in progress. There are still - and perhaps always will be - those who seek to divide rather than unite.”

The causes of the current spike in tensions in Northern Ireland are diverse, but spring from familiar roots.

One issue is the opposition among dissident republicans, who seek a unified Ireland, to the designation of Londonderry as a United Kingdom City of Culture in 2013.

The name of the city is itself contentious, with Roman Catholics referring to it as Derry, its name before British authorities changed it to Londonderry centuries ago.

Last month, a prison officer, David Black, was shot dead in Northern Ireland and a republican splinter group said it committed the killing to protest conditions at a jail where guards carry out strip searches on detained republican dissidents.

The discovery of the devices Thursday came after days of apparently unconnected protests by unionist groups.

The passionate debate over the flags reflects a broader battle of symbols that delineate the feuding republicans and unionists and, in part, sustain their confrontation.

The unionist protests spread to other parts of Northern Ireland, illustrating the enduring and emotive pull of sectarian passions despite the 1998 Good Friday agreement that cemented peace after 30 years of conflict, known as The Troubles, in which more than 3,500 people were killed.

During her visit, Clinton spoke with political leaders at Stormont, the seat of the power-sharing Northern Ireland administration, and visited a new center devoted to the Titanic, the doomed ocean liner built in Belfast and launched in 1911 before it sank on its maiden voyage after striking an iceberg in 1912.

Front Section, Pages 2 on 12/08/2012