At least 100 people were injured in street battles that started on Tuesday after stone-throwing protesters attemped to break through barbed wire barricades around Zia's office.
At least three crude homemade bombs - tin pots filled with explosives - went off at the scene before police responded by firing tears gas shells.
Mohammed Nasim, a former interior minister, and other senior opposition Awami League members were among those injured.
Others were taken to hospital with head wounds.
Police had imposed a ban on rallies and marches around Zia's office in the capital after the opposition alliance announced plans for Tuesday's protest.
The authorities had deployed at least 10,000 policemen and paramilitary troops around Zia's office to prevent the protest.
Police assault on the rally left
many protesters wounded
Streets in the area were closed to public vehicles, causing huge traffic jams.
A 14-party alliance led by the Awami League is demanding electoral reforms before January's national elections.
The parties want the replacement of the election commissioner and his two deputies, accusing them of being partisan.
Tofail Ahmed, an opposition spokesman, said: "This fascist government is using the police to attack peaceful demonstrators.
"What we have been demanding is good for democracy."