Kelly's $100k bond -- 10% of his $1 million bail -- was finally posted Monday after he appeared in court and entered a not guilty plea to all 10 counts of aggravated sexual abuse. As he walked out of jail, wearing his blue down coat, Kelly was swarmed by photogs and reporters.

There also seemed to be a number of protesters yelling at Kelly, calling him a pedophile and asking how he'd feel if someone urinated on his daughter -- but they used more colorful language.

Sources in Kelly's camp tell us he paid the bond with a bank cashier's check ... so, there won't be any issues with his check clearing.

Although Kelly's out, he's staring down another deadline. TMZ broke the story ... he has to pay about $160k in back child support to his ex-wife, Drea Kelly, by March 6. If he doesn't, it's right back to County lockup.

The Kelly sources tell us he plans to make arrangements to pay the child support by next week, so he can focus on his criminal defense ... and avoid jail.

It's possible he could be facing additional charges after attorney Michael Avenatti dropped an additional bombshell -- a NEW tape showing Kelly having sex with the same 14-year-old girl from the other tape. Avenatti has now turned over both tapes to Cook County prosecutors.

5:05 PM PT -- R. Kelly made a pit stop after leaving jail, stopping off at McDonald's for some quick eats.

We're told Kelly ordered a quarter pounder meal with BBQ sauce on the side, washing it down with a soda and some coffee. We're told Kels got cream and sugar with his coffee, but he went back to the counter and asked for more.

We're told the singer sat down and ate with his team inside the fast-food restaurant for about half an hour, and they left after a small crowd began to gather.