Police say at least 49 people have been killed in political and ethnic violence in Pakistan's southern port city of Karachi in the last three days.

Gunmen opened fire on buses Thursday, with more than 22 people killed in the latest unrest throughout the city.

Police blame clashes between rival political groups in Sindh province, the Muttahida Qaumi Movement (MQM) and the Awami National Party (ANP).

The MQM largely represents the Urdu-speaking community, and until last month was part of the ruling coalition in Sindh. ANP represents ethnic Pashtuns.

MQM leader Raza Haroon on Thursday threatened a city-wide strike and said provincial lawmakers would rally Friday to protest the government's inaction to quell the violence. He said MQM supporters were being targeted because the party quit the coalition.

Amnesty International called on Pakistani authorities to thoroughly investigate targeted killings, not just in Karachi, but throughout Pakistan. Amnesty said Thursday the increase in violence and “general insecurity” in the country over the past two years reflects a “grave law and order crisis” in Pakistan.

The Human Rights Commission of Pakistan says 490 people were victims of targeted killings in Karachi in the first half of this year.

Additional police and paramilitary personnel have been deployed in the city to try and quell the latest surge in political violence.

Some 16 million people live in Pakistan's economic hub of Karachi. The city has also been the scene of sectarian violence between Sunni and Shi'ite Muslims and militant attacks.