Dozens of Liberian police officers have been wounded in clashes between the regular force and the body guarding the main port in the capital, Monrovia.
UN peacekeepers quelled the violence
Liberian National Police spokesman Alvin Jask said the clashes came after his officers went to the seaport to investigate reports of fuel theft.
He said the police director was taken hostage by the seaport police, who threw stones at her reinforcements.
The fighting was quelled by UN troops - deployed after a 14-year war.
The BBC's Jonathan Paye-Layleh in Monrovia says this is the first clash between rival security forces since Ellen Johnson-Sirleaf took power last year, following historic elections.
Mr Jask said 22 police officers were in hospital - 18 with serious injuries.
Deputy Port Managing Director Reginald Pratt said at least 12 of their workers had been injured in the attack and they were being treated at clinics around Monrovia.
Eyewitnesses say the reinforcements called by police director Beatrice Munah Sieh, moved into the port compound, assaulting port workers and smashing doors.
Our reporter says blood trails could be seen in some places where the early morning clash took place.
Ambulances shuttled between the port and hospitals with the wounded as hundreds of curious people stood watching outside the port fence.