"I only distributed the anti-colonialism pamphlets that I had printed in school. What kind of crime had I committed?" Tsang asked.

Internet users last week mocked Tsang's past after Ng warned students they could jeopardise their future if they joined the pro-democracy movement.

Occupy Central intends to rally 10,000 protesters to block Central streets if the government does not come up with a satisfactory plan to implement universal suffrage for the 2017 chief executive election.

Lawmakers and the Professional Teachers' Union condemned Ng for putting political pressure on schools.

Meanwhile, pan-democratic heavyweight Cardinal Joseph Zen Ze-kiun urged Hongkongers to take part in the public vote between June 20 and 22 to choose their preferred reform plan from three proposals shortlisted by Occupy Central supporters earlier this month.

Zen was speaking amid a split within the pan-democratic camp over the upcoming vote. The moderates had expressed concern that all three proposals involved allowing the public to nominate chief executive candidates - an idea that Beijing has repeatedly rejected as going against Basic Law stipulations.

What mattered was not which proposal won the "referendum", but the total number of people who cast their vote, he said.

"We are not asking you 'which proposal will succeed'; we are asking 'do you want genuine universal suffrage'," Zen said. "The total votes collected would be the sum of the people who call for universal suffrage."

A low turnout at the public vote would be an excuse for those who were against democracy to deny Hongkongers universal suffrage, he added.