Bus enthusiasts marked the end of the New World First Bus brand by joining the final fleet of vehicles in its colours in the early hours of Saturday, as the company's merger with Citybus officially took effect.

In sending off the 25-year-old brand, the operator put on five special buses on route 970X from Cheung Sha Wan to Aberdeen.

A parent who brought his bus loving son for the last ride said: "We are witnessing the end of an era."

Another enthusiast said he had taken photos of the automated announcement system's LED screen, which read "Welcome aboard New World First Bus".

One of the bus captains who drove the last journey said he was touched by the number of people bidding farewell to the brand.

"Some passengers who brought the next generation here told me that they used to take the bus I drove," he said.

The merger also saw the first bus under the merged firm's new Citybus branding depart Island Resort in Siu Sai Wan at 4am.

A Citybus general manager, Roger Ma, said hours later that the first day of operations has been smooth.

"The coming Monday will be the first work day faced by the new Citybus franchise. We will be fully prepared and deploy extra manpower to monitor our services and passenger load, with the aim of providing services smoothly," Ma said.

The two loss-making operators announced their merger last year.

Their parent company, Bravo Transport, said the merger would give the operator more flexibility to enhance existing services or introduce new services, and would not result in higher fares. It's pledged to invest HK$3.5 billion in the business.