Reflecting Kabila's persistent popularity among many of Congo's nearly 68 million people, tens of thousands of government loyalists held a counter-demonstration a day after Tshisekedi's return, calling for the president to stay on.
Protests against a potential poll delay have in the past turned violent and authorities arrested dozens of critics of Kabila in April, although last week's rallies were peaceful.
Foreign donors fear political tensions could easily lead to armed conflict -- Congo's mix of ethnic strife and foreign interference driven by competition over its fabulous mineral wealth has bloodied it for two decades.
A regional war between 1996 and 2003 in east Congo -- a treasure trove of rare metals used in phones and other gadgets -- killed millions of people, spawned countless militias and sucked in soldiers from half-a-dozen African countries.
Sunday's protest was still peaceful by 2.30 p.m. (1330) GMT, watched by armoured police trucks. Some of those gathered said they would try to force Kabila out if necessary.
"We want Kabila's immediate resignation. If he uses force, we will respond with force," said David Slim, 24, a trader.
Elsewhere in the crowd, someone waved a white cross with the words: "Adieu Kabila, RIP."