By the monument to the Ukrainian poet Taras Shevchenko, they came with yellow-blue flags and chanted "Glory to Ukraine" and "Down with the Russian occupiers". Across town by the monument to Vladimir Lenin, the flags were red, white and blue, and the chanting was for union with Russia.
There were two very different visions of Crimea's future on display at the two rallies in its capital on Sunday, a week before the peninsula holds a referendum on joining Russia, which the West has called illegitimate, but Russia's parliament has strongly suggested it will honour.
At a similar pro-Ukraine rally in the port city of Sevastopol, the demonstrators were attacked by a group of whip-wielding Cossacks, in a forewarning of the possible violence in the coming months. There are fears that after the referendum, there could be clashes between the large pro-Russian population and the minority Crimean Tatar and ethnic Ukrainian populations, who are aghast at the prospect of union with Moscow.
Watch: Tensions mount as rivals stage rallies in Ukraine
The referendum, to be held on Sunday, will ask Crimeans if they want more autonomy within Ukraine or union with Russia. However, the local parliament has already voted for union with Russia and said the referendum was merely meant to "confirm" the decision.
Russian President Vladimir Putin said last week that there was no suggestion of Russia annexing Crimea, but Moscow put on a warm reception for the region's de facto leaders on Friday , and in a phone call with the British prime minister, David Cameron, and the German chancellor, Angela Merkel, on Sunday, Putin appeared to back the referendum.
The region is now full of heavily armed pro-Russian militias, backing Russian military actions in the region. But even among the ethnic Russians, who make up more than half of Crimea's residents, there is not a consensus on joining with Russia. Many would prefer enhanced autonomy within Ukraine.
At the pro-Ukraine protest, 62-year-old Larisa said she was filled with worry at the prospect of joining Russia. "I am Russian, I was born in the far east of Russia, but I am a Ukrainian patriot. We are pawns in Putin's game. Who is he to say we need defending? He has sent in troops to our country on the pretext of protecting us, but from whom?"
Other Russians, however, were certain that union with Russia was the only thing that could save the peninsula from being attacked by the new government in Kiev, which is widely described here as fascist.
"Our grandfathers fought the Nazis, and now they are in tears looking at these revolting fascists in Kiev," said Vladimir, a factory worker from the town of Bakhchisarai who plans to volunteer for a local self-defence unit.
In Kiev, there was also a rally near the Taras Shevchenko monument, celebrating the 200th anniversary of the national icon's birth.
"We face the biggest challenge for our country and nation for the history of modern independent Ukraine," Arseniy Yatsenyuk, Ukraine's prime minister, told the crowd. Yatsenyuk will travel to Washington this week and meet US President Barack Obama at the White House tomorrow. In Kiev, he insisted that Ukraine would never give up Crimea to Russia.