Sam Sen Baku, head of the Civic Constitutional Project, told the EFE news agency that he had been informed by telephone calls from Kaduna that at least 300 persons had been killed in the religious fighting. The actual number of deaths could be even higher but remains difficult to confirm, he added.
The violence began Monday when Muslims attempted to halt a march by thousands of Christians who were protesting a proposal by Islamic leaders to implement Sharia, or Koranic law, in the city.
Muslims are the majority in Kaduna, one of the most important cities in central Nigeria. Kaduna city is the capital of Kaduna state.
Armed with sticks, knives and iron rods, gangs of Christians and Muslims fought hand-to-hand through the city streets and burned homes and businesses.
Authorities deployed police reinforcements in Kaduna but the radio stations reported that tension remained high.
The most-populated country in Africa with 120 million people, Nigeria frequently suffers violence between its many ethnic and religious groups.
United Press International reported sources as saying police fired indiscriminately on crowds of rioters.
Sharia law was imposed in October in the neighboring northern state of Zamfara, and several other small, rural, predominantly Muslim states have declared their intention to follow suit.
But Kaduna is larger and has a more diverse population, a human rights worker told the BBC.
So far, the governor of the state has set up a committee to study the proposal but has made no firm commitment to impose Sharia law. Muslim advocates of the change say the laws will apply only to Muslims.
In Zamfara, Islamic punishments meted out so far have included lashes for drunkenness and extramarital sex, and jail terms for taxi drivers who pick up women.
The Kaduna state government ordered shops and schools to close Tuesday, and the local authorities appealed for calm.
The Christian protests follow weeks of demonstrations by Muslims demanding the introduction of Islamic law.