A spokesman said the deployment to all of the country's 64 districts and about 500 sub-districts would be completed later in the day.
Rights groups have criticised the plan saying army officers could abuse their power to arrest.
Thousands of people attended the rally near the presidential palace in Dhaka amid tight security.
Security forces took position at several entry points to the rally venue, and on rooftops of surrounding buildings and two stadiums.
On Tuesday, police using batons and tear gas struggled to drive back activists hurling stones, bricks and homemade bombs.
"Why can't people in this country resolve their differences in a civilised way instead of resorting to violence and anarchy?"
Shafiq, Dhaka, Bangladesh
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At least 100 people were injured and dozens detained by police, witnesses said.
"Smoke from the homemade bombs and tear gas shells covered the area, with people running for cover," one witness said.
The injured included 20 leaders of different parties. About 230 people have been injured in clashes during the past three days.
Abdul Jalil, general-secretary of the Awami League, said they would defy all bans to implement their protest.
He called on alliance leaders and workers to resist the elections and urged voters to boycott the poll.
Local media has reported that the opposition is planning non-stop blockades during election day.
Reiterating an earlier ban, police said that anyone causing disruption would be "dealt with severely" but they were braced for more violence as the alliance said it would block roads leading to the palace of Iajuddin Ahmed, the president, who heads the caretaker administration.
Hasina has accused Ahmed of bias and demanded that he and election commissioners step down before elections are held.