Pakistani officials say unknown gunmen opened fire on a rally in Karachi Tuesday, killing at least 10 people — including women — and wounding 20 others.

Sindhi nationalists, including the Awami Tehrik political group, had launched the rally late Tuesday afternoon to protest graffiti that appeared in recent days in Karachi and nearby Hyderabad. The wall writings demanded a separate province for the area's Urdu-speaking community.

No one immediately claimed responsibility for Tuesday's violence.

Following the shooting, rally organizer Ayaz Latif Palijo blamed disgruntled elements within the Urdu-speaking community for the violence. He also accused Karachi's dominant MQM political party — with consists mainly of Urdu speakers — of being behind the newly-emerged separatist movement and called for another strike Wednesday.

MQM officials have denied being behind the graffiti.

Crowds of angry Pakistanis set several vehicles on fire after the shooting, causing businesses to shut down across the city of 18 million people.

Pakistan's largest and most commercially important city, Karachi has long been the site of sectarian tension between the different language communities. The Human Rights Commission of Pakistan says some 400 people have died in sectarian violence in Karachi this year alone — a higher death toll than in all the terrorist attacks across the country. Nearly 1,800 people died in violence in Karachi in 2011.