Dozens of police with helmets and riot shields tried to push back hundreds of protesters at one end of the tunnel, television footage showed.
One or two were hit with batons, the South China Morning Post reported. Protesters speaking to AFP after the police had dispersed also said they had been pepper sprayed.
Protesters told AFP that they had decided to take the highway after police earlier cleared another occupied main road without warning.
"The government refuses to talk to us so we will keep occupying the roads until we get a real dialogue."
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As police dropped back, the protesters began to move metal barricades inside the tunnel, blocking it to traffic.
The road, which runs east-west outside the government complex, had not been occupied before.
Huge crowds have intermittently rallied against China's insistence that it will vet candidates standing for election as the semi-autonomous city's next leader in 2017 -- a move protesters have labeled as "fake democracy".
While the activists have been praised for their civility and organisational skills, they have also brought widespread disruption.
Police had been keeping a low profile at the three main protest sites after a decision to fire tear gas at peaceful demonstrators on September 28 caused outrage and encouraged tens of thousands to take to the streets.
But in the last two days, officers have begun probing protester defences in raids aimed at opening some roads to traffic, while allowing the bulk of demonstrators to stay in place.
Police had vowed earlier today to tear down more street barricades manned by pro-democracy protesters, hours after hundreds of officers armed with chainsaws and boltcutters partially cleared two major roads occupied for a fortnight.