AFP: Bahrain released at dawn on Saturday a Sunni opposition leader who was jailed more than four years for involvement in Shiite-led anti-government protests, his party announced.

The Waed secular group said on its official Twitter account that party chief Ibrahim Sharif was freed.

Bahrain, home to the US Navy's Fifth Fleet, has been rocked by unrest since security forces crushed Shiite-led protests in 2011 demanding a constitutional monarchy and an elected prime minister.

Sharif had played a prominent role in the month-long protests and was later among a group of 20 activists tried for plotting to overthrow the Sunni rulers of Shiite-majority Bahrain.

In June 2011, eight Shiite activists were jailed for life, and the rest for two to 15 years. Sharif received a five-year sentence.

The judgement sparked concern in Washington which criticised the "severity" of the sentences and the use of military tribunals to try civilians.

Sharif's release comes four days after a Bahrain court jailed prominent Shiite opposition leader Ali Salman for four years for inciting disobedience and hatred in the kingdom.

Salman heads Al-Wefaq Shiite political association, one of the key forces behind the 2011 protests demanding reforms in Bahrain, and his arrest in December had drawn US and Iranian condemnation.

At least 89 people have been killed in clashes with security forces since 2011, while hundreds have been arrested and put on trial, rights groups say.

Protesters continue to clash frequently with security forces in Shiite villages outside Manama.