More than 1.1 million people are now affected by tougher restrictions on home visits after they were extended to two more areas in the west of Scotland.
The measures came into force in East Dunbartonshire and Renfrewshire at midnight after a rise in cases.
The rules had already been reimposed in Glasgow city, West Dunbartonshire and East Renfrewshire last week.
First Minister Nicola Sturgeon said acting quickly now could "stem the tide of transmission" in the area.
But she has warned that there is a "definite trend" of rising case numbers across Scotland.
Measures were reimposed in parts of the greater Glasgow area last week in response to a rise in coronavirus cases.
People are being told not to host people from other households inside their own homes, or visit another person's home.
Meetings in pubs and restaurants and outdoor areas are still permitted - although the Scottish government said the hospitality sector would be monitored in the coming days to see whether restrictions should be extended.
A further 78 positive cases were reported in the Greater Glasgow and Clyde health board on Monday.
Ms Sturgeon said it was "too early" to say whether the fresh lockdown had had any effect on cases.
The measures are targeted at household meetings, with Ms Sturgeon saying it was "still the view of public health teams that the significant factor driving transmission is people meeting up in their own homes".
She said local authorities in the area would "pay close attention to hospitality" and would encourage people to act responsibly while using bars and restaurants.
The widened restrictions - which also mean there should only be "essential" visits to people in hospitals and care homes - will be reviewed in a week's time.
Ms Sturgeon said the measures are not yet being extended to Lanarkshire or Inverclyde - noting that levels of infection were "significantly lower" in Inverclyde.
The restrictions will now apply to apply to 179,000 people living in Renfrewshire and 108,000 in East Dunbartonshire.
The city of Glasgow has a population of 633,120, while there are 95,530 people in East Renfrewshire and 88,930 in West Dunbartonshire.
Ms Sturgeon said it was "regrettable we are in this position", but that the measures banning household visits were "considered proportionate but also the most effective".
She added: "If we act quickly and preventatively now, we can stem the tide of transmission and avoid having other restrictions put in place."
The first minister had earlier warned that a continuing rise in Covid-19 cases in Scotland could see her government "put the brakes" on the planned easing of some restrictions.
An average of 152 positive tests have been recorded each day over the past week - compared to 14 per day six weeks ago.
The number of hospital admissions and deaths has not risen as sharply, although Ms Sturgeon warned that "this may just be a matter of time".