Nine hydrants were set off leading to disrupted water supplies in Drumchapel, Milton, Robroyston, Stepps, Easterhouse, and Ibrox.
Soaring temperatures this week - it was 27C in Glasgow yesterday - have seen the hydrants deliberately opened, sending jets of water into the air.
Youngsters then play in them to help to cool down.
Scottish Water received around 40 calls from affected customers in a two-hour period.
And with high temperatures expected to continue, there are fears more hydrants will be set off.
The disruption to supplies started at around 6pm yesterday.
Engineers were working to repair the damage and restore supplies.
A Scottish Water spokesman said: "As the result of vandalism to fire hydrants, customers may experience loss of water supply, low pressure and discoloured water.
"We are working closely with the emergency services to restore the supplies."
Police Scotland, the Scottish Fire and Rescue Service, and Scottish Water, previously urged people not to set off fire hydrants.
Fire chiefs say the vandalism could hinder its officers in the event of a blaze.
Tampering with hydrants can cause flooding and disrupt supplies to homes.