About 50,000 South Koreans have rallied in the capital Seoul in protest over the deaths of two teenage girls killed by US servicemen in a road accident.

Several thousand riot police prevented the protesters from marching on the US embassy.

The BBC's Caroline Gluck, in Seoul, says it was the largest demonstration since the two soldiers, whose armoured vehicle hit the girls, were acquitted of negligent homicide by a US military court last month.

It was the largest protest since the soldiers were cleared by a military court

Protesters want the soldiers retried in a South Korean court and are demanding changes in an agreement that governs the rights and conduct of about 37,000 US troops in the country.

Another demand appeared to have been met when US President George Bush expressed his regret over the deaths in a telephone conversation with South Korean President Kim Dae-Jung on Friday.

But his words failed to appease the crowd which marched through Seoul on Saturday.

Many called for a public apology.

"We cannot accept it as a direct apology to the Korean people. We demand him to apologise in the capacity of the US president instead of whispering personal sadness on the phone," said protester Chai Hee-Byeong.

Carrying candles in mourning, demonstrators cheered and applauded when speakers demanded more South Korean jurisdiction over US troops.

There were also angry words for President Kim Dae-jung, who was called a "US puppet".

Large US flags were held aloft by the crowd and torn apart.

Base entered

In other protests, two students in the city of Daegu, 320 kilometres (200 miles) south-east of Seoul, broke into a US military base and climbed on to a 30 metre (100ft) water tank.

There were no reports of violent clashes or casualties.

White House spokesman Ari Fleischer said Mr Bush had "pledged to work closely with the South Korean Government to prevent such accidents in the future".

Mr Bush said the American people cherished their strong alliance with South Korea and President Kim Dae-jung had said South Koreans appreciated the peacekeeping role of US troops, Mr Fleischer added.

The current Status of Forces Agreement (Sofa) currently requires that American soldiers charged with crimes while on duty in South Korea be tried in US military tribunals.

Our correspondent says the government has opened talks with US officials to improve the code, but their efforts so far have not calmed the public mood.