Jiang died in his home city of Shanghai just after noon on Wednesday of leukemia and multiple organ failure. He was 96.
A date has yet to be set for his funeral.
The front page of the ruling Communist Party's official People's Daily devoted its whole front page to Jiang, and carried a large picture of him wearing his trademark "toad" glasses.
"Beloved comrade Jiang Zemin will never be forgotten," it said in its headline, above a story republishing the official announcement of his death.
Flags flew at half mast on key government buildings and Chinese embassies abroad, while the home pages of e-commerce platforms Taobao and JD.com also turned black and white.
But foreign governments, political parties and "friendly personages" will not be invited to send delegations or representatives to China to attend the mourning activities, the official Xinhua news agency said.
At one of the largest foreign banks in China, employees have been asked to wear black in meetings with regulators, senior staff have been asked not to be photographed at parties and the bank has put marketing activities on hold for 10 days, a senior executive at the lender told Reuters, speaking on condition of anonymity as they were not authorized to speak to the media.
Some Chinese social media users have posted pictures and videos of Jiang speaking or laughing and articles about his 1997 speech at Harvard University in English, reminiscing about an era when China and the West were on better terms.