More than 20 people were killed in Guinea on Monday in clashes between protesters and security forces who opened fire on the most violent day of a two-week general strike against President Lansana Conte, witnesses said.

Authorities arrested union leaders in a bid to break the crippling strike that unions had called against the ageing Conte, saying he was unfit to rule.

The witnesses said police and soldiers initially fired in the air to try to disperse thousands of protesters marching in the West African country's dilapidated capital Conakry, but then some turned their guns on the crowd.

At least 150 people were wounded in the latest violence.

A doctor at the Donka Hospital in Conakry said 17 people were killed by gunfire in the capital, while a senior local government official said six other people were killed in the north-eastern towns of Kankan and Siguiri.

"I've never seen a situation like this since I've been a doctor. The majority of the victims appear to have been directly shot at," said the doctor, who asked not to be named. Gunfire was heard outside the hospital as he spoke.

After a day of shooting and protests across the capital, police said they had arrested the leaders of the two main unions which had launched the strike against the president, who is a reclusive, ailing diabetic in his 70s.

A senior police officer said Ibrahima Fofana of the Guinean Workers' Union (USTG) and Rabiatou Sira Diallo of the National Confederation of Guinean Workers (CNTG) were detained for meddling in politics and "incitement to revolt".

At least 15 other union officials were arrested, including some seized by armed presidential guards led by Conte's son, Ousmane, who burst into CNTG headquarters, witnesses said.

While most of the deaths appeared to have occurred when police fired to disperse protesters, witnesses said two people were killed when police shot at one group trying to break into a shuttered shop in the Madina neighbourhood.

Riot police sealed off the city centre. Trucks sped by carrying arrested youths guarded by armed soldiers.

The strike turned violent last week and although casualty figures have been confused, at least eight people had already been killed in clashes between protesters and police.

United Nations Secretary General Ban Ki-moon expressed concern about what he called the "excessive use of force".

"The Secretary General calls on the government of Guinea to exercise maximum restraint on its security forces, and urges the parties to engage in dialogue in order to find a peaceful resolution to the dispute," his spokesperson said in New York.

The strike has disrupted the mining industry in Guinea, the world's biggest exporter of bauxite from which aluminium is extracted.

The latest unrest flared as religious leaders, union chiefs and government representatives struggled to negotiate a solution to the strike, which has paralysed public transport and shut government offices, schools, shops and markets.

Strike leaders say Conte, who has ruled Guinea since seizing power in a 1984 coup, has become increasingly erratic.

They cite repeated scares about his health, sudden and chaotic cabinet reshuffles and his personal intervention to free from jail two former allies accused of graft.

Conte appealed on Sunday for the people and the army to unite behind him.#

He has offered some concessions and sacked his closest government ally in a bid to appease the strikers, but union leaders say this is not enough.

"This has become a popular uprising. We're not in control of it," said union negotiator Ousmane Souare. - Reuters