BANGKOK (AFP) - An unexploded hand grenade was discovered outside a Democrat Party office in Bangkok on Tuesday, police said, adding to tensions in the capital now beseiged by Thai opposition protesters, who have swarmed several more ministries in Bangkok on Tuesday to try to topple the government, as Prime Minister Yingluck Shinawatra faced a no-confidence motion in Parliament and warned against "mob rule".

Demonstrators ralling against Ms Yingluck and her brother, ousted premier Thaksin Shinawatra, vowed to stay on the street overnight, said they had moved to cut off the water supply to the building and were planning to sever the electricity.

Protest spokesman Akanat Promphan said the protesters would wait until Wednesday before making a "big move". Authorities said demonstrators had ended their occupation of the foreign ministry as they focused on other targets.

"We don't dislike Thaksin - we loathe him. This is the end for him," said Ms Dhiranut Bunna, a 43-year-old housewife from Bangkok who was among the protesters still at the Finance Ministry.

Apart from the Interior Ministry, most government buildings taken over had only a light security presence outside.

But security has been tightened on Bangkok's streets since the expansion late Monday of the Internal Security Act, which gives authorities additional powers to block routes, impose a curfew, ban gatherings and carry out searches.

A Thai court on Tuesday approved an arrest warrant for protest leader Suthep Thaugsuban - who resigned from the opposition Democrat Party to head the rallies - in connection with the occupation of government buildings.

"I ask him to surrender, otherwise police can arrest him on sight," said Colonel Sunthorn Kongklam of Bang Sue police station in the capital.

"Suthep is not trying to throw out the government... he wants to throw out democracy and replace it with an ultra-royalist administration," red shirt leader Thida Thavornseth told AFP.

Both the United States and Britain raised concerns over the rallies.