Sudanese workers are refusing to go to work in an attempt to pressurise the ruling military government to make way for civilian rule.
The opposition called for a campaign of civil disobedience including a national strike, which began on Sunday.
Four people were killed after security forces fired tear gas and live ammunition.
Monday remained quiet in Khartoum although some businesses started to reopen and a few buses were running.
In another development, three rebel leaders have been deported from the city to South Sudan.
One of the three, Yasir Arman, had only returned to Sudan last month after years in exile after he was sentenced to death in absentia.
Most shops, markets and banks in the capital, as well as in several other cities, remained closed on Monday as staff followed instructions from the Sudanese Professionals Association (SPA), the pro-democracy opposition, to not attend work.
The SPA called the strike after more than 100 peaceful protesters were killed by a paramilitary group, the Rapid Support Forces (RSF), on 3 June.
"The civil disobedience movement will begin Sunday and end only when a civilian government announces itself in power on state television," the SPA said in a statement.
"Disobedience is a peaceful act capable of bringing to its knees the most powerful weapons arsenal in the world."
Protesters have set up roadblocks across the capital. Social media users with access to a connection reported that the country's internet was blocked by the ruling military government.
What's the background?
The military ousted long-time President Omar al-Bashir in April following months of protests against him. A military council promised a transition to civilian rule.
But pro-democracy campaigners say the military council cannot be trusted after Monday's crackdown against a sit-in demonstration in Khartoum - and they have rejected an offer of talks.