The BBC's correspondent in Karachi, Ilyas Khan, reports that most public transport was off the road on Wednesday morning, but almost back as normal by the end of the day.
Differing explanations
Witnesses say the trouble escalated after a rally to support Mr Niazi - staged by lawyers allied to the Karachi-based regional party, the MQM - was attacked by lawyers from other parties.
Police say that MQM supporters in the old part of Karachi retaliated by taking to the streets and torching cars, offices and businesses.
They also say that six people were killed when rioters set fire to lawyers' chambers in a building adjacent to the city court in downtown Karachi.
The police add that those burned to death included a woman.
Our correspondent says that it is difficult to get a clear and independent picture of what exactly happened during Wednesday's violence because all the parties accused of being involved - the MQM, the Pakistan Muslim League-Nawaz (PML-N) and the Pakistan People's Party (PPP) - have put forward differing explanations.
He says that it is, however, clear that Karachi is at its most tense since violent clashes between the MQM and other parties last year.