KUWAIT CITY — Police on Sunday extended the detention of a Kuwaiti arrested last week for allegedly burning an Iranian flag in protest at a Shiite tweeter's apparent insult to the Prophet Mohammed, a legal source said.

Nahar al-Hajeri, who was arrested on Thursday, was questioned and then sent to the central jail for three weeks pending further investigation and trial, the source told AFP.

A foreign ministry official on Thursday voiced "rejection and condemnation of this irresponsible action."

The official said the action, "made by a deviant group," is considered a "breach of the laws and norms and greatly undermines the distinguished relations between Kuwait and its (Shiite) neighbour Iran."

Dozens of activists led by Sunni Islamist MPs gathered outside the offices of the secret service police where Hajeri had been detained, demanding his release.

The original protest was staged on Wednesday night by Sunni activists and several MPs a day after authorities arrested a Shiite alleged to have made remarks on the micro-blogging website Twitter that were deemed offensive to the prophet, his wife Aisha and some companions.

The Shiite, identified in media reports as Hamad al-Naqi, was in custody pending further investigation amid a frenzy of condemnation by Sunni preachers and MPs, some of them demanding the death penalty for the man.

Shiite MPs also condemned the alleged insults.

Sectarian tensions have flared in Kuwait between the Sunni majority and Shiites, who form about a third of the native population of 1.17 million, reflecting rising regional tensions between the two Islamic sects.