It says a lot for the art of Ai Weiwei that his installation Sunflower Seeds has continued to move and fascinate visitors to Tate Modern even after health and safety issues caused it to be roped off. Denied the intimacy of walking over millions of porcelain replicas of seeds and running handfuls of them through their fingers as the artist had intended, people stare instead from the Turbine Hall bridge, or walk alongside the grey carpet of myriad seeds. It is curiously beguiling, like looking into Monet's waterlily paintings, except what daunts the mind here is not endless reflection in calm water but the thought of how many tiny fragile things you are looking at, and the notion that each represents a soul, a person, a life. A life that could bloom into a sunflower, but is instead frozen forever as a monochrome seed.

Now, Ai Weiwei is being treated by the Chinese police as if he were one more nameless sunflower seed to crush underfoot. An individual of international fame and potent charisma, he seemed unassailable, but presumably that is the point of detaining him – to stamp out the idea that any individual is greater than the law of the state.

Ai Weiwei was not apparently connected with the call for a "Jasmine revolution" that is believed to have provoked the current crackdown in China. Yet this terrific artist has not been afraid to put his criticisms of the government in explicit language. Last year he wrote in the Guardian urging David Cameron on his visit to China to speak up for democratic rights and insist that "the civilised world cannot see China as a civilised country if it doesn't change its own behaviour". "I don't believe that these are western values," he added. "These are universal values."

In 2010, this trenchant declaration that democracy is a universal human right – that it is not only for western countries but for all countries – stood out a mile from the run of political discourse. This year, exactly the same call for universal human rights and democracy has transfixed Arab nations, with the same bold rejection of the doublespeak that has in the past led to one-party states being excused or tolerated. No wonder China's communist party is scared.

Ai Weiwei joins a select band of artists who have risked everything for ideals. Michelangelo was arguably the first dissident artist when he created fortifications for the revolutionary republic of Florence in 1529: in fact, he and Ai Weiwei have something in common as artists who work on a grand public scale. You could call the floor of the Tate Turbine Hall a modern equivalent of the Sistine Chapel ceiling, in both fame and impact. In the 19th century, the tough realist painter Gustave Courbet joined the Paris commune and died in exile for his ideals: again, like Ai Weiwei and Michelangelo, he was a charismatic personality who seemed too big to be brought down. But he was cruelly punished.

Will Ai Weiwei be a Courbet or a Michelangelo? While the Communard painter was ruined by his political enemies, Michelangelo was spared and allowed to carry on working and enjoying his success after the defeat of the Florentine rebellion – he really was too big to hurt. We have to hope that, once it feels it has made its ugly, bullying point, the state will release Ai Weiwei and his fame will continue to protect him. Whatever happens, he is that rare thing: the artist as moral and political hero.