The incident occurred shortly after 9am in the southbound lane at kilometre marker 28-30 before a bridge crossing over the Tha Chin River in Muang district, according to the traffic news outlet FM91 Trafficpro.
Two people were found dead and five injured, according to Pol Lt Pirote Pinthip, traffic inspector at the Muang Samut Sakhon police station, FM91 Trafficpro reported at 10am.
Local news reports said the collapsed parts of the crane, which was being used for the expressway bridge construction, struck two vehicles travelling below.
Police and rescuers were working at the scene to assess the damage. They cordoned off the area and closed some traffic lanes on both the outbound and inbound sides for safety, reports the Bangkok Post.
As a result, traffic congestion was building up and motorists were advised to avoid the route.
Rama II Road is the main and most direct route connecting Bangkok with the southern region and is known for frequent accidents related to the prolonged construction of an expressway above it.
The accident comes just a day after a construction crane fell onto a moving train in Sikhiu district of Nakhon Ratchasima province, killing at least 32 people and inuring 64 others.