BELFAST, Northern Ireland (CNN) -- At least 20 people were dead and dozens wounded in a bomb attack in the town of Omagh in Northern Ireland on Saturday, police said.

No group claimed responsibility for the blast near a crowded shopping district of the town west of Belfast. But the placement of the bomb -- near Omagh's courthouse, seen by some as a symbol of London's rule in the British province -- suggested that a dissident republican guerrilla group may have planted the bomb.

Some observers said it could have been carried out by the Continuity IRA or the Real IRA, who refused to follow the Irish Republican Army's cease-fire and continue to violently oppose British rule.

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The latest on the Omagh bombing from Peter Taggart of Belfast Radio   336K/29 sec. AIFF or WAV sound 224K/17 sec. AIFF or WAV sound

Police said there had been a telephone warning of a bomb less than an hour before the explosion.

The caller said the bomb was directly outside the courthouse, prompting security forces to move the people away from the building. But then the blast occurred closer to an area of shops, pubs and supermarkets -- which was exactly the area that the evacuees were moved towards.

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"Police moved everybody away from this area to where the bomb really was," said Nigel O'Kane, owner of a pub nearby the courthouse. "They cleared everybody away towards Market Street, then about 20 minutes later the bomb blew up behind them."

"It was awful -- people running around crying. Nobody could believe it. People (were) panicking for their loved ones. You just hope there's as few people injured as possible. There was a couple buildings flattened by the bomb."

Several outlawed paramilitary groups continued their violent attacks, despite the April 1998 peace agreement signed by all major factions on both sides of the sectarian and political divide.

The Omagh attack happened on the 29th anniversary of the British government's decision to deploy troops in Belfast as peacekeepers between Protestants and Catholics. That decision triggered the rise of the modern-day IRA.

Last month, 35 people were wounded in the mainly Protestant town of Banbridge, southwest of Belfast.

Earlier in June, a car bomb wrecked the mainly Protestant section of Newtownhamilton, also near Belfast.