A chain of bomb blasts have hit three of the most popular tourist resorts as well as towns in southern Thailand on Thursday and Friday, killing four people and wounding dozens, days after the country voted to accept a military-backed charter in a referendum.

Four bombs exploded in the upscale resort of Hua Hin, about 200 km (125 miles) south of Bangkok on Thursday evening and Friday morning, killing two people and wounding at least 24.

Other blasts hit the tourist island of Phuket, a resort town in Phang Nga province, and Surat Thani, a city that is the gateway to islands such as Koh Samui in Thailand’s Gulf.

Hua Hin is home to the Klai Kangwon royal palace, which translates as “Far from Worries Palace”, where King Bhumibol Adulayadej, the world’s longest reigning monarch, and his wife, Queen Sirikit, have often stayed in recent years, until both were hospitalized.

Friday was a public holiday in Thailand to mark the queen’s birthday, which is celebrated as Mother’s Day.

No group has claimed responsibility, though suspicion could fall on groups fighting an insurgency in Muslim-majority provinces in southern Thailand.