* Evacuation of financial hub is biggest in post-war Germany
* Police will use force if necessary to ensure evacuation
* Fire official underscores bomb's destructive power (Recasts, adds details, updates reporting credit)
FRANKFURT, Sept 1 (Reuters) - Frankfurt city officials have warned that Germany's financial capital could grind to a halt on Monday if residents don't heed orders to vacate their homes to allow the defusing of a massive World War Two bomb.
On Sunday, the city will evacuate some 60,000 people in the nation's biggest such manoeuvre since the war while officials disarm the British bomb discovered on a building site this week in Frankfurt's leafy Westend, where many wealthy bankers live.
Fire and police chiefs, at a hastily called press conference on Friday, said they would use force and incarceration if necessary to clear the area of residents.
An uncontrolled explosion of the bomb would be big enough to flatten a city block, Frankfurt fire chief Reinhard Ries told reporters.
"This bomb has more than 1.4 tonnes of explosives," he said. "It's not just fragments that are the problem, but also the pressure that it creates that would dismantle all the buildings in a 100-metre (yard) radius.".
The HC 4000 bomb is assumed to have been dropped by Britain's Royal Air Force during the 1939-45 war. Such finds are not unusual, but rarely are the unexploded bombs so large and in such a sensitive position.
The compulsory evacuation radius of 1.5 km (roughly a mile) around the bomb includes police headquarters, two hospitals, transport systems and Germany's central bank storing $70 billion in gold reserves.
Officials on Friday called on Frankfurt's residents to clear the area by 8 a.m. on Sunday and warned the effort could take at least 12 hours.
Police said they couldn't begin defusing the bomb until they were sure everyone had left the area. They would ring every doorbell and use heat-sensing technology from overhead helicopters to help them identify stragglers, they said.
Roads and transport systems, including the parts of the underground, will be closed during the work and for at least two hours after the bomb is defused, to allow patients to be transported back to hospitals without traffic.
Air traffic from Frankfurt airport could also be affected if there is an easterly wind on Sunday, air traffic control told Reuters on Friday. Also, small private planes, helicopters and drones will be banned from the evacuation zone, they said.
Frankfurters can spend the day at shelters set up at the trade fair and the Jahrhunderthalle convention centre, police have said.
In addition, most museums are offering Frankfurt residents free entry on Sunday, and a few of them will open their doors earlier in the morning than usual, the city said on its website. (Reporting by Tom Sims and Maria Sheahan; Editing by Toby Chopra)