Black-shirted activists were hauled away minutes before a National Day flag-raising ceremony, as other protesters reported clashing with police after being blocked from public areas.
Radical lawmaker "Long Hair" Leung Kwok-hung described police arrangements as "particularly bad" and said it was difficult for his group to get to Golden Bauhinia Square in time for a ceremony marking China's 63rd founding anniversary.
Shortly before 8am, security guards led away about 10 people in black shirts, including four members of anti-national-education group Scholarism, from the square where Chief Executive Leung Chun-ying and around 2,500 officials and citizens had gathered.
"As a Hong Kong citizen, why can't I see the national flag-raising ceremony?" one youngster shouted while being taken away.
Scholarism's Lee Chun-kwok said they did not chant or put up placards, but officers claiming to be from the Home Affairs Department told them to leave 10 minutes after they arrived.
"We asked which clause of Hong Kong's law empowered them to do it. They didn't reply," Lee said.
Tong Wing-po, a home affairs senior liaison officer, said: "The rules of the flag-raising ceremony say anyone who disturbs the process may be asked to leave."
When asked what constituted a disturbance, Tong replied: "I have nothing to add."
About 20 League of Social Democrats and April 5th Action protesters who marched to the square in Wan Chai accused the police of stalling their progress.
"We began at 7.15am and the march should have taken about 15 minutes. But we arrived at around 8.05am," Leung said, adding that they were forced to march in a walkway less than one metre wide.
"The police arrangement was particularly bad this year."
At the end of the chief executive's speech at a cocktail reception in the Convention and Exhibition Centre, next to the square, the radical lawmaker shouted: "End one-party dictatorship. Give power back to the people."
The legislator was immediately ejected from the venue.
Another Scholarism contingent led by convenor Joshua Wong Chi-fung clashed with police as they marched from Harbour Road to the square.
Police put up a banner warning them not to proceed, saying the protesters had not applied for a permit.
The activists kept chanting, "Let us go" and "Withdraw brainwashing national education now".
In the afternoon, dozens of people waving the old British colonial flag demonstrated outside the central government's liaison office, advancing a separate identity as "Hongkongers" and not "Chinese".