By Nick Hawton BBC correspondent in Belgrade

On the road to the airport, every few hundred metres, there is a policeman or police car. Many of the officers are carrying assault rifles.

Many Serbs feel Mr Djindjic's death is a great loss

There is no curfew and shops and businesses are open as usual, but there is no mistaking that Belgrade, and Serbia, is under a state of emergency following the assassination of Prime Minister Zoran Djindjic.

But the police presence did not stop a large queue forming to sign a book of condolence close to where the assassination took place. There was an air of depression and resignation among those waiting.

"I really loved him," said one lady in the queue. "We have lost a special person. I feel like I have lost a member of my family."

One man said Serbia had lost a great deal.

"A huge strength has been lost for the process of reform," he said. "I don't know if there are any other politicians who can replace Mr Djindjic."

Public sorrow

Throughout the city and throughout the country, flags have been lowered to half mast. State television has been playing classical music, interspersed with the latest news reports on the hunt for the killers.

Security has also been stepped up on Serbia's borders. All cars are being searched and identification papers checked. Suspicion and tension are written on the faces of the border guards.

At a news conference in Belgrade, a visibly shaken Interior Minister, Dusan Mihailovic, said the government would continue Mr Djindjic's reform programme.

"As a human being, I'll do everything to avenge this death," he said. "As a minister, I assure you, we will catch those responsible."

In the meantime, preparations are underway for Mr Djndjic's funeral. It will take place on Saturday in Belgrade's central cemetery. He will be buried in a special area reserved for prominent people from Serbian society.

Thousands of people are expected to attend. Many will want to show their appreciation for someone they believe had tried to drag Serbia out of the dark days and into a new future.

Late in the afternoon, it began to snow in Belgrade - unseasonable weather, but somehow reflecting the mood.