Transport users in London are facing a day of disruption as thousands of bus drivers stage a 24-hour strike.
About 5,000 Metroline and First drivers started their industrial action at 0300 BST. The Unite union wants a standard £30,000 salary for all bus drivers.
The walkout, due to end overnight, affected around 160 bus routes mainly across north-east and west London.
A spokesman for First called the demands "utterly unrealistic" and said at least 10% of its buses were running.
Metrobus drivers had also been due to join the strike but the company won a last-minute legal challenge to prevent its staff from walking out.
The 24-hour strike follows strikes in August and September on the same issue.
Unite officer Peter Kavanagh said yearly pay in the capital's 18 bus companies varied by up to £6,000.
The union is intent on dragging further bus companies into strikes in pursuit of its fruitless bid to standardise conditions
Adrian Jones, First
"There is a startling disparity between bus drivers' pay in the capital, with rosters in some companies seeing many drivers complete nearly 60 hours a week," Mr Kavanagh said.
He also called on London Mayor Boris Johnson to get involved in resolving the dispute.
Unite is also balloting workers in other bus companies for industrial action later this month.
Adrian Jones, managing director of First, said: "The union is intent on dragging further bus companies into strikes in pursuit of its fruitless bid to standardise conditions of working and pay in all companies."
He said First's bus drivers were among the best-paid bus workers in London.
"Between 2003 and 2007, First's bus drivers' pay increased by 34%, during which time inflation has increased by only 16%," Mr Jones said.
"Most drivers receive over £25,000 a year and this rises to over £30,000 a year for a normal rostered week for over a quarter of drivers."
Unite plans a further 24-hour strike on 22 October.
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