Passengers were showered with glass after several windows on four buses were shattered when "heavy objects" were thrown at them.
The Stagecoach East guided buses were targeted at about 18:00 GMT on Monday near Longstanton in Cambridgeshire.
Nick Beveridge, a passenger on one bus, said there was "the most tremendous bang" as two windows "caved in totally, shattering glass everywhere".
The bus operator said the matter had been reported to the police.
Mr Beveridge said: "Shortly before we reached Longstanton there was the most tremendous bang - I thought there'd been an explosion.
"Two of the windows... caved in totally, shattering glass everywhere, into the hair of the lady sitting opposite me and on to the spare seat next to me.
"I can only praise the diligence of the driver - his first concern was whether anyone was injured."
The bus was taken to the nearby park and ride site.
Morning, does anybody know what caused all the broken windows on the buses traveling on the busway last evening?— Shawn Stephens (@Stephens01Shawn) November 19, 2019
"I heard two bus drivers speaking and that the bus had had something heavy thrown at it," Mr Beveridge said.
He said the double decker bus behind theirs "had also suffered damage to one of its windows".
A spokeswoman for Stagecoach East said: "There was some vandalism on the busway on Monday... between Histon and Longstanton.
"Four buses had windows smashed after items were thrown at them."
She said no-one was injured in the incidents, but the vandalism had "some impact to services [on Tuesday morning] whilst vehicles are being repaired".