A matatu stage [bus stop] is one of the busiest, loudest, most confusing places a stranger to Nairobi can find themselves. Thousands of locals jostle for position to navigate the city's notorious rush hour. But shortly after 8pm on Monday (24 October), was a different story. The usually crowded Kaka terminus in central Nairobi was almost eerily quiet. Camouflaged soldiers lingered, hands on guns, talking quietly amongst themselves. A few metres away, security men redirected traffic and prevented interested bystanders from entering the terminus-cum-crime scene.

Monday saw the second grenade attack in Nairobi in less than 24 hours, but the first fatal one. Eyewitness reports described how the grenade was thrown at a full matatu, but bounced off and landed in a crowd of people attempting to enter various vehicles. The grenade exploded as it hit the ground, killing one and injuring thirteen others, mostly below the waist. Commuters took the full brunt of the blast. It came barely 18 hours after a man hurled a grenade into a busy Mwaura's nightclub on Mfangano Lane, injuring 14 people.

It is only three weeks since I took a drink in Mwaura's, a very local venue visited by working class Kenyans on account of its cheap alcohol. The place was little more than a dive, a rundown crevasse in the wall where ordinary Kenyans drank and socialised. And that is the theme of these attacks thus far. Though the US embassy warned last week that they had "credible" information that bars and shopping malls frequented by westerners could be targeted, the sites of the two grenades have been soft targets used by local Kenyans on a regular basis. While security has been stepped up in the more upmarket venues, it is almost impossible to adequately police local bars, markets and transport hubs. Concerned expatriates have been able to hide behind metal detectors and armed security guards, but local people going about their local business now fear for their lives. The relatively small period of time between attacks suggest those responsible are going for maximum impact.

This effect was seen yesterday morning, with fewer than normal making it into work on time or at all, presumably fearing a repeat performance. Nairobi is on high alert, and the fear is there for all to see. After Kenyan troops entered neighbouring Somalia last week, a military plane in the skies of Nairobi drew a banner declaring: "Defending Kenya". But what has become clear is that, though the police and Red Cross were on the scene with extraordinary speed on Monday, there is no protecting Nairobi's people when the perpetrators of this terror can strike anywhere at any time.

Who is responsible is still open for debate. Al-Shabaab, the Islamist terror organisation that Kenyan troops are seeking in Somalia, have not claimed responsibility as they have for previous attacks in the region. In the wake of last week's invasion, al-Shabaab leaders promised that Nairobi would experience real terror if Kenyan troops did not leave Somalia. It presently seems more likely that Kenya-based Somali sympathisers with al-Shabaab are behind the attacks, acting in accord with the public statements of the group's leaders rather than from direct orders. This was the topic of much whispered conversation on Monday evening.

Nairobi has a large Somali population, many of them students living in the Eastleigh area. Nicknamed "little Mogadishu" because of its high population of Somalian residents, it is now braced for a crackdown that will affect innocent Somalis as well as those with al-Shabaab connections. "We wake up prepared for a day and a time like this," said one resident. "When world events will shape the daily livelihoods of the hundreds of thousands living in this little corner of Nairobi." Comments on the Daily Nation's report of the blasts included vitriol such as "It is very difficult to trust a Somali" and "This is the time the non-Kenyan Somalis be removed from Kenya". Reports of police patrolling the area asking for identity cards have already surfaced. Arrests have occurred in Malindi and Nakuru. It is probable that the difficulties of properly identifying al-Shabaab sympathisers will foster an atmosphere of resentment against Somali residents in Kenya.

Nairobi today lives in fear. My taxi driver home told me that "people are scared, and people will be scared to go out in darkness". Security has been stepped up across the city, but nobody can escape the feeling that a repeat of yesterday's violence is possible anywhere, at any time.

Tom Jackson is a British journalist working for a news agency in Nairobi.