The CPB and Bangaldesher Samajtantrik Dal (BaSad) enforced the half-day shutdown protesting against the latest fuel price hike.
A future plan of agitation was likely to be announced at the rally.
A plain-clothes policeman started using pepper spray just after the rally began, bdnews24.com correspondent Golam Mujtaba said from the spot.
After the incident, CPB chief Mujahidul Islam Selim said they would intensify their movement if the offending policeman was not brought to book within 24 hours.
Police sprayed coloured water from water canons at the rally. They baton-charge the gathering and used pepper spray on the strikers.
As the strike supporters tried to gather and bring out a procession, the law enforcers used water canon again to spray hot water.
BaSad General Secretary Khalekuzzaman condemned the use of pepper spray on the strikers. “The pepper spray is used on mad dogs and ferocious animals in foreign countries while police is using it on those who are observing peaceful protests for the sake of the people.”
On Jan 6, the opposition alliance had observed a countrywide general shutdown on similar demands.