The biggest show in town last week starred 30,000 stressed-out City workers, and computer systems they all dreaded crashing
The revolution was invisible. The streets were empty and the wine bars closed as the City of London seemed to nurse a hangover on New Year's Day.

That was an illusion. Something extraordinary was happening behind closed doors, and odd little signs gave the game away: the silver Porsche parked on a yellow line; a man carried bags of sandwiches into an office that looked deserted.

The race was on to be ready for the euro. The champagne corks that had popped in Brussels the day before had sounded starting pistols in the Square Mile. There was nothing for the public to see, but the most dramatic weekend in the modern history of the City had begun.

For once the leading players were not the traders, those glamour boys and girls of finance, who were in an end-of-term mood as they watched the televised announcement of euro conversion rates on Thursday.

"There was a little frisson of excitement, but not much, because the figures were as expected," said Chris Tucker of Barclay's Capital. "There was hardly any trading. They just had to wind up their business for the end of the year." Once that was done they were finished for the day, free to find the nearest wine bar and practise the words to Auld Lang Syne.

Instead, the pressure was on - and the party off - for computer and systems staff at the 600 banks and financial institutions in the City.

After years in the shadows of their flashier colleagues, the techies were about to move centre stage, and the challenge before them was immense. They had to close the books on 1998, change an estimated pounds 1.6bn in assets from the 11 newly obsolete currencies into euros, install new trading systems and test the lot - all in 72 hours.

Failure to be ready in time would blow a hole in London's claim to be the most important money market in the world, where a third of all foreign exchange deals were carried out. Bankers in European cities, in particular Paris and Frankfurt, were also working feverishly so that they could be ready to steal some of that business if London faltered.

Most banks had been preparing for conversion weekend for three years, with numerous fraught rehearsals. There was no overall co-ordination, but the total cost of the work was thought to be around pounds 2bn. With so much at stake, around 30,000 staff were asked to work in shifts over the holiday.

Barclays called 500 into its offices at Canary Wharf, and a further 200 at Poole in Dorset, the site that pumps money around the bank's various British outlets. There were said to be 1,000 people at work at HSBC, and 400 each at Deutsche Bank and Goldman Sachs. Salomon Smith Barney in Victoria kept 200 on over the weekend, and booked 70 rooms at hotels near the building. "They can leave their desks and be asleep in half an hour," said a spokesman.

THE BANK of England had 300 people at work, but kept normal weekday hours. As well as converting its own systems, it had teams on standby to advise other institutions and keep watch on the stability of the whole financial system while it was made ready to begin trading euros. "There's not much time for stuff to go wrong and be corrected," said Sally Reid, a spokesperson for the Bank, on Thursday. "Nobody wants their particular institution to be the one that can't make the grade."

The most unpopular shift of the weekend kept workers in the office over midnight on New Year's Eve. Not that they got much sympathy from the traders assembled at the Lamb Tavern in Leadenhall Market to enjoy a couple of quick pints of Winter Warmer before heading home to party.

"They've known this was coming for a long while," said Guy, who worked for an international bank and was drinking with friends from the Lloyd's building next door. "They're ready. If they mess up - God forbid - we'll all pay the price."

The cost to his company if they didn't make it might be measured in billions of pounds and hundreds of jobs. "Anyway, they'll get bonuses for working tonight," Guy added. "It's not worth whinging about."

No one would have dared whinge at the American investment bank Merrill Lynch, where the party line was pride at taking part in history. To pep up the troops around midnight Mitchell Shivers, global head of EMU preparation, made a quick tour of the workplace "just to say hello and wish everyone a Happy New Year". There was no champagne but he promised they would "party hard" when it was all over.

There were 200 people on duty through the night, many of them in the command centre, where electronic flow charts flashed up the state of the systems as they were reconfigured. The television was switched to business news, but in a smaller room next door was another set with a stack of videos.

"On the overnight shift there can be quiet periods as the machines do their work," said Jane Richardson, the EMU project director for Europe. "So people take it in turns to make coffee, nibble some food, relax. We keep going with an awful lot of coffee and Diet Cokes. You've got to pace yourself. It's a bit of a marathon, this one."

Everyone had dressed down, said Ms Richardson, who was wearing an immaculately pressed denim shirt and jeans, with lots of jewellery. She lived 10 minutes away by car at the Butler's Wharf complex at Tower Bridge, but other colleagues were staying at the Barbican and Royal Horseguards hotels. "If people have a one-hour commute that is wasted time, whereas if you've only got to go 10 minutes to the hotel, people can get five hours' sleep. It is important to consider individuals and not to burn them out."

A family help-desk had been set up so partners and children could contact their loved ones, who were working away from their normal desks and might not be home in days.

"This has been so long coming, and it is so historic," Ms Reid added. "There's such a buzz about just getting it done, that people thought, `OK, we'll celebrate New Year later'."

Of all the people in Britain on duty over the weekend City workers were the most lavishly prepared. Like most City firms, Merrill Lynch had set up an extra electricity generator and made plans against fire, burst water mains and evacuation. The canteen offered hot food around the clock. Coaches had been hired to take workers to hotel rooms. Other companies took block bookings with taxi companies: the cost of keeping a black cab on stand-by for eight hours was pounds 350.

THE CORPORATION of London had paid around pounds 7,000 for the Waterloo & City Line to stay open. London Underground operated a weekend service, although there were few passengers at Bank early on New Year's Day, and those at Liverpool Street were mostly weary revellers waiting for trains out of town. The police were not operating the usual check-points, although many private cars were used. Parking meters were free and yellow lines ignored.

There were balloons and signs saying Happy New Year outside the Fleetwood pub, where cheerful staff had obviously shaken off the effects of their own merriment the previous night. It was a shame so few customers had turned up. Corney & Barrow opened a bar on New Year's Day for the first time in its 228-year history, but might have wondered why it bothered. The branch at Broadgate Circle offered restorative Bloody Marys with hot roast beef sandwiches, but was as empty as the ice rink it looked down on. Everywhere else was closed.

This morning thousands more are expected to make their way into the City, as traders and senior managers arrive to review the conversion and plan for tomorrow. Even as you read these words there may be pandemonium in one or more of the office blocks as systems go haywire, but such disasters will be kept secret for as long as possible, invisible behind the glass and concrete facades.

That can't last. The markets reopen this evening, in Sydney and then Tokyo. At four or five in the morning the rest of the vast City workforce will begin to arrive. By the end of the first day's trading - if not before - the outside world will know who was ready for the new era and who, if anyone, is covered in euro-shame. Jane Richardson of Merrill Lynch shuddered when asked what would happen if her own team failed. "That," she said coldly, "is not an option."