Chief Executive John Lee said on Sunday that customs clearance on the first day of the resumption of quarantine-free travel with the mainland has been smooth, adding that he believes the 50,000 quota for crossing the border is adequate.

Besides the 50,000 places now available each way at land crossings, about 10,000 spaces for flights, ferry trips or bus crossings of the bridge to Zhuhai are also available daily.

Speaking to reporters after visiting the Lok Ma Chau check point, Lee said the quota was not fully booked yet over the Lunar New Year, but authorities would closely monitor the situation to see if there's a need to review the numbers.

"As of today, the quota isn't fully booked. Even for Lunar New Year's Eve, there's still about a thousand left on the quota," Lee said.

"I'll monitor the situation and liaise with Guangzhou and Shenzhen authorities, to see when we can review the quota."

Up to 11am on Sunday, some 9,000 Hong Kong residents were reported to have crossed the border at Lok Ma Chau.

Up to 35,000 can now head north each day via that crossing, with the same number of mainlanders able to travel in the opposite direction.

The border crossings at Shenzhen Bay and Man Kam To were also reopened on Sunday.

Hong Kong people began queueing up at checkpoints to use the crossings to the mainland for the first time in three years from early Sunday morning.

One woman surnamed Lee was among the first to cross.

"I want to go to the mainland to visit my father," she told RTHK.

"He's 99. So I took the first train to Guangzhou. I'm really grateful that the government decided to drop the quarantine."

The Secretary for Security, Chris Tang, said that about half of Sunday's quota had been booked up.

Chief Secretary Eric Chan said on Saturday that there were still plenty of spaces left for Hongkongers wanting to travel to the mainland in the coming weeks despite a rush of bookings since registration opened on Thursday.

He revealed that almost 400,000 people had secured places from Sunday until March 4. The number of bookings had reached 410,000 by 6pm on Saturday, 48 hours after registrations opened.

Shenzhen authorities said on Saturday evening that 1,600 people had registered to cross at the Futian checkpoint on Sunday, which connects to Lok Ma Chau.