(Releads with new death toll)

By Abdi Sheikh and Aweys Yusuf

MOGADISHU, April 20 (Reuters) - Somali Islamist insurgents and government troops exchanged mortar fire on Sunday and a prominent human rights group said 81 people had been killed in the past 24 hours in some of the heaviest clashes in months.

The fighting was fiercest in the Islamist stronghold of northern Mogadishu where the government and its Ethiopian allies are trying to flush out the remnants of a sharia courts movement ousted from the capital at the end of 2006.

"Eighty-one people were killed and 119 were wounded in the violence in Mogadishu since Saturday," Sudan Ali Ahmed, chairman of the Elman Peace and Human Rights Organisation, told Reuters by telephone.

He said he had collated the death toll from local hospitals, undercover activists counting bodies in the street and families.

There was no independent verification of the death toll, but residents had reported escalating clashes since Saturday.

"We condemn the unceasing fighting and the use of artillery on the civilian population," Ahmed said. "We also condemn the opposition groups who fight among the civilians and use them as (human) shields."

His group estimates that 6,500 people were killed last year in Somalia's conflict and 1.5 million uprooted from their homes.

The interim administration is struggling to contain a deepening Islamist-led insurgency involving near-daily attacks on allied Somali-Ethiopian troops.

The Islamists have also carried out an increasing number of hit-and-run attacks on towns -- seizing control from local administrations that often amount to little more than militias, only to melt away before government reinforcements arrive.

HEAVY FIGHTING

Residents said the two sides had strengthened their positions overnight and exchanged heavy fire in the early hours of Sunday around the Save Our Souls (SOS) Hospital.

"A mortar shell landed on a house just behind SOS hospital, killing an old man and seriously wounding his wife and her 3 children," said a medical worker who declined to be named.

"As we were running to help this family we saw an unidentified dead man lying on the ground," he added.

Health workers in various hospitals said they were treating scores of patients wounded in the fighting.

One witness said he saw the bodies of four men near the main livestock market, adding that no one had dared to take the bodies away "because the whole place is under Ethiopian siege."

Prime Minister Nur Hassan Hussein expressed regret for Somalis forced to flee the violence but said his interim government and its Ethiopian allies had the right to self-defence.

"I am very sorry for the poor civilians who evacuate when fighting takes place," he told a news conference.

"The government of Somalia is always ready for peace, but if our troops and Ethiopian troops are attacked, fighting with any group that is against peace will be inevitable." (Additional reporting by Mohamed Abdi; writing by Katie Nguyen; editing by Tim Pearce) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ )

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