Kano is the fourth and largest Nigerian state to adopt Sharia - which includes punishments such as amputation and flogging - and other states are likely to follow.
The issue has provoked fear among northern Nigeria's minority non-Muslim population and sparked violent unrest.
People began gathering from dawn, chanting Islamic slogans and the dusty streets around the parade ground were congested with excited young men.
Elderly residents of Kano said they had never seen such a large crowd.
Kano Governor Rabiu Kwankwaso said on Wednesday that although Sharia had been adopted, its actual implementation would not begin for several months whilst the necessary structures were put in place.
Troops
Supporters of Sharia say it will bring peace to Kano and force corrupt politicians to reform their ways, but there is anxiety among Christians.
The authorities have given assurances that Sharia will not apply to non-Muslims, but many Christians have already abandoned Kano city, which has a history of religious violence.
Troops and riot police have been deployed to strengthen security.
Hundreds of people died in clashes in February and May between Christians and Muslims in neighbouring Kaduna state after it attempted to introduce Sharia.
The Kano State government said last week it was implementing Sharia law in response to the demands of the people of the state.
But our correspondent says the sheer size of Kano's population, its commercial significance and its history of sectarian violence all make this a particularly sensitive moment in Nigeria's history.
Christian withdrawal
The Christian minority in the north consists largely of migrants from the Christian-dominated south.
Many shops and businesses are boarded up.
Banks said there had been a massive run on deposits because of withdrawals by customers leaving the state.
A spokesman for the independent committee that called for the introduction of Islamic law, Dr Datti Ahmed, accused anti-Islamic elements of spreading propaganda designed to frighten non-Muslims into leaving Kano.
He said the Sharia and national legal systems would operate side by side.
Nigerian Muslim leaders have said all along that the Islamic law would not affect the lives of the Christian minority.
But the BBC correspondent says the move is bound to be viewed with apprehension by the Nigerian federal government, which has been struggling to build unity in the huge and diverse country.
Ethnic and religious violence in Nigeria has increased since President Olusegun Obasanjo took office a year ago, ending 15 years of military dictatorship.
Islamic law is a broad code for living for all Muslims - including prayers, fasting and donations to the poor.
But it also includes a penal code, including stoning for adulterers and amputation for thieves.
In private matters, non-Muslims are exempt, but if they commit a public crime like theft, they too could be subject to it.