The deadline expires on Wednesday for foreign private security firms in Afghanistan to hand over control to Afghan government forces.
The move follows increasing frustration over alleged high-handed behaviour by some foreign security guards.
The change will also rein in Afghan security companies, but it has raised fears that foreign aid workers will be be less well protected in future.
Nato say that the move will provide thousands of jobs for Afghan guards.
The BBC's David Loyn in Kabul says that already there are far fewer armed security guards visible on the streets where most foreigners live.
Some foreign companies with offices in the city have an extension to keep their guards for 40 days.
But apart from diplomatic missions which will be able to keep their own guards - from Wednesday all foreigners will have to pay for guards from a new Afghan Public Protection Force.
The force is currently 6,000 strong, far smaller than the 40,000 private security guards now operating.
Our correspondent says that some Afghan security companies operated like private armies, getting rich on contracts to protect aid convoys.
President Hamid Karzai has railed for years against the large number of "guns-for-hire" in Afghanistan, arguing that private security companies skirted the law.
Nato said that the move could keep billions of dollars of foreign money in Afghanistan that would otherwise go to private security companies.