He has been known as a chancellor that was difficult to find when the going got tough. But yesterday, just days after entering Number 10, Gordon Brown had to show the leadership qualities that may help define his premiership and ultimately decide if can go on to become an elected prime minister.

After chairing two meetings of his government's emergency contingencies committee, named after the briefing room where the meeting are held in the Cabinet Office, Cobra, Brown crossed to Downing Street to make his appeal in front of television cameras to urge the British people to be "united and resolute".

After an attack in London that didn't take hundreds of lives simply because of luck rather than good intelligence, and then an attack on Glasgow Airport yesterday which could have taken scores of lives if the doors of the terminal building hadn't held, Brown knew he had to sound re-assuring, confident, Churchillian even. These are qualities Tony Blair could conjure up when required.

However, Gordon Brown is not Tony Blair. But he did as best he could to turn his reputation as "iron chancellor" into steel-cored prime minister.

He said "The first duty of the government is the security and safety of all the British people, so it is right to raise the level of security at airports and in crowded places in the light of the heightened threat."

Brown had just taken Britain's national security threat to its highest level - critical.

The new prime minister has wanted his first 100 days in office to be marked by radical reforms, reforms he's been planning since Blair announced this year would be his last. But if the two attacks in London and Glasgow are even slightly related, there are others who wanted his premiership to begin with not one, but two massacres.

His appointment of Jacqui Smith as home secretary, the inexperienced but calm chief whip, was intended to display a new tone on security matters. But now Smith, like Brown, has no choice. Calm is good, but determination and resolve will be called for.

Brown urged the British people to be vigilant, saying: "I want them to support the police and all the authorities in the difficult decisions that they have to make. I know the British people will stand together, united, resolute and strong."

When an analytical calm does eventually arrive, there will be one issue that Brown knows he will have to deal with - Iraq.

Questions will be asked on how these men were radicalised: what drove them to try to carry out mass slaughter? The war in Iraq, led by the US and backed by Britain, helped turn Iraq into an Islamic cause that has been exported to Madrid, to London and now to Glasgow.

This weekend was supposed to be his opportunity to relax after the momentous events of last week. He has learned, sharply, that as Britain's prime minister, leadership has its own timetable.

As he turned and walked away from the television cameras in Downing Street he looked less like a chancellor and more like a Prime Minister, one suddenly aware of the responsibility he shoulders.