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Before he knew what was happening, Charles Ngeana, 31, at the scene of Nairobi's 18 November blast, was struck by a flying sheet of metal. The roof of a packed 'matatu' minibus on Route 28 was lifted up and its right side completely blown out.

"I ran at first because of the dust," said Ngeana. "The impact was very big."

Ball-bearings, used to maximize the impact of an explosive device, left nearby vehicles speckled with holes. "It definitely came from inside the matatu," Ngeana said. It took him a few moments to realize he had gashes on his face, throat and right leg.

Soon after the blast, which took place in Eastleigh, the commercial centre of Nairobi's ethnic Somali population, fighting broke out in the street.

People appearing to be of Somali origin were attacked for their perceived association with Al-Shabab, a Somali insurgency group that has repeatedly targeted Kenya because of its military involvement in Somalia. In the melee, Ngeana sought the protection of a Kenya Red Cross Society (KRCS) emergency response team.

The bomb exploded around 4pm.

Security deteriorating

"We anticipate this on Saturdays and Sundays," said Abdi Nasir, the technical director of the National Blood Transfusion Service, speaking outside the Kenyatta National Hospital where 29 casualties of the blast had arrived. They were followed by over 10 casualties of the riots.

"The previous Sunday, and for around one month, every Sunday there has been a disaster. So we were waiting, going by the current trend."

With just four months until elections, the security situation in Kenya is deteriorating. The country has suffered more than such 20 attacks this year, most of them involving grenades or small explosive devices; the attacks have been blamed on Al-Shabab or its sympathizers.

This is the worst attack to hit Nairobi, in terms of casualties, this year. Hundreds of people filled three streets afterward, throwing stones and fighting.

One man stabbed in the neck during the post-blast riot said: "They were shouting 'these people, they're the ones who are our enemies', so I tried to run away."

"It was Somali people fighting Kenyan people," he said. He and his family had been coming from church and had no knowledge of the blast. When their car was held up, they decided to walk. "We found ourselves in a group of Somali youths - after a few minutes, two men came with a knife. They tried to kill me," he said.

Continuing violence

Street battles involving men and women armed with machetes and other weapons broke out again on 19 November on the main road in Eastleigh, where several shop windows were shattered by stones.

When a group of youths destroyed a bus, workers on a building site started throwing rocks, which lead to a bloody battle between the two groups.

Police reported two serious injuries, but witnesses said the figure is probably higher. With shops looted and burned, the riots were eventually dispersed by police with tear gas.

Preparing for the worst

Tension between ethnic Somalis and Kenyans has escalated since the Kenyan incursion into Somalia.

KRCS has put a disaster response team on standby in case of further violence.

"When a blast happens, there is a call for crowd control, and it is then that we get a little bit pushed," said Jervis Sundays, a KRCS communications officer, who nonetheless says his team is unfazed by explosions. "We have handled a number of blasts," he said.

Although the regularity of attacks and their repercussions mean that emergency response teams are currently well prepared, stocks of blood have been depleted. "These disasters are really attacking the blood bank," said Nasir. "We are appealing on a daily basis."

KRCS has set up a tent in central Nairobi for blood donations. Kenya's National Blood Bank usually has supplies sufficient to treat around 100 casualties, but with incidents in which a high proportion of injuries are burns and severe tissue trauma, patients often need more than one unit of blood.

"It seems to be getting worse as the days go by. The blood we have will not be used to address leukaemia or anaemia - instead the focus is moving onto casualties and addressing disaster," said Nasir.

KRCS has also responded to recent mass-casualty situations far from the capital. Its staff helped in a rescue and recovery operation following a 10 November ambush in the northern Samburu District, in which more than 40 police officers were killed by suspected cattle-rustlers. KRCS has also played a leading role in the humanitarian response to inter-communal clashes in the coastal Tana River District.