Bahrain's king has ousted four Cabinet members as a prominent opposition leader returned from exile and urged the country's rulers to back up reform promises with action.

Thousands of demonstrators pressed their demands for change by marching on government buildings in the capital, Manama.

Two members of the royal family were among the replaced Cabinet members, a possible nod to protesters' complaints that the ruling house of Al Khalifa holds too much control over state levers of power.

Bahrain's prime minister - the king's uncle, whose more than four decades in power has made him a focus of protesters' calls for reform - remains in his post.

Meanwhile, the return from self-imposed exile of Hassan Mushaima, a senior Shiite figure, could mark a new phase for an anti-government movement in the tiny Persian Gulf nation that is a strategically important American ally.

He heads a Shiite group known as Haq, which is considered more hardline than the main Shiite political bloc that has helped drive two weeks of protests.

His speaking appearance hours later, preceded by bursts of fireworks, electrified an already enthusiastic crowd packed into Pearl Square, the centre of demonstrations.

Mr Mushaima, who had been among a group of Shiite activists previously accused of plotting to overthrow Bahrain's rulers, called on the government to be more responsive to protesters' demands for far-reaching political reforms.

"Dialogue... is not enough. Promising is not enough. We have to see something on the ground," he told reporters.

Asked if he hoped to lead the protest movement, he said: "I'm always saying to the people 'I'm your servant'."