I knew we were heading in the right direction, because of the convoy of ambulances speeding the other way, ferrying the wounded to hospital. As we approached, it suddenly became clear that this was a major attack in which many, many people had perished.

On the central barrier of the highway, I saw the first of the bodies: A bearded man, lying frozen on the floor, his hands clasped together, his mouth slightly open, his bare feet black with dirt.

As I walked forward, into the knot of traffic and bystanders, I started to see more bodies, some lying face down, some carefully arranged in rows. And then I noticed the body-parts. There was a foot, a scrap of scalp, with hair matted in blood and a glistening pile of intestines.

At the epicenter of the bombing, rivulets of blood were running across the road, ambulance sirens competed with each other, making conversation impossible. My feet scrunched on broken glass, and then slipped on human flesh.

There was an acrid, overwhelming smell of explosives. The wounded were still being loaded on to stretchers by harassed paramedics. It was chaotic and horrifying. There were still hundreds of PPP supporters standing and watching, some shouting into their mobile phones, others helping to move the dead.

In the center of the road, the blackened skeleton of a burnt out car smoldered. Next to it, a badly damaged police truck, and then 20 feet away, was the bus that had been carrying Benazir Bhutto. It was burnt and peppered with shrapnel, the windscreen had been cracked and the driver’s cab was damaged.

Benazir’s photo stared out from the side, watching the awful carnage impassively.

We'd been watching this motorcade earlier in the evening, and I remember thinking what an easy target it represented. It was moving at a walking pace, mobbed by thousands of people. There were nowhere near enough police to hold back the crowds. An attack seemed inevitable. All that surprised me was that it took so long for them to strike.

A full 10 hours after she landed, the carnival of Bhutto’s return had suddenly been transformed into a wretched, blood-soaked tableau.

In a lengthy press conference the next day, Benazir Bhutto tried to strike a defiant tone, talking about fighting for democracy and freedom, and resisting the extremists.

But her critics are furious at her refusal to heed police warnings and scale back her procession.

The editorial pages of the Dawn newspaper in Karachi raise similar questions. "It must be asked of the PPP leaders: Was the slow crawl necessary?" asks the main editorial.

A letter printed opposite puts it more bluntly: "Why was Ms. Bhutto allowed to proceed, putting so many lives in jeopardy? Was it so important to make a display of public strength for the benefit of western benefactors on whose nod Ms. Bhutto was returning, that the massacre of hundreds of people was considered an acceptable loss?"

These are questions which may damage her standing as the countdown begins to January’s parliamentary elections.

But Benazir Bhutto seems determined for her campaign to go ahead amid this taut atmosphere, with the very real threat of further attacks.

-- From Dan Rivers, CNN International Correspondent.