At least 43 people died and 76 were injured in three car bombings in the centre of the Iraqi capital, Baghdad.
The bus station was crowded when the blasts happened
Two of the blasts went off within 10 minutes of each other at 0800 (0400 GMT) at the busy Nahda bus station.
The third blast happened on the road to a nearby hospital some 15 minutes later, just as victims of the first two attacks were being brought in.
Four men were arrested at the bus station on suspicion of being involved in the bombings, officials said.
No details have been given about the suspects, who are now being questioned.
All three attacks were less than 30 minutes apart, and police say their apparent co-ordination and planning means casualty figures are almost certain to rise.
The blasts were the largest attacks by insurgents in recent weeks.
Attacks have dropped off amid negotiations on a new Iraqi constitution, which reached a deadlock this week.
Police and officials investigating the attacks were trying to determine whether the blasts were work of suicide bombers.
The bus station serves various parts of the country and would normally have been crowded with travellers at the time of the attacks, says the BBC's Mike Wooldridge in Baghdad.
One eyewitness told Reuters news agency they saw a coach blow up.
The blast destroyed buses as well as police and civilian vehicles
"We heard an explosion in the garage, we went there and ran towards the buses for Kut, Basra and Amara," said Ahmed Jabur.
"A coach blew up. When we were leaving, another one blew up in the middle of police cars."
Most of the victims appeared to be civilians, many of whom were trapped on buses.
Shortly afterwards, the third car bomb echoed around the city. Medics and police helping the wounded to hospital were among the injured, reports say.
In other violence:
- Two US soldiers died in separate attacks in southern Baghdad, and northern Iraq, the US military says
- Six Iraqi soldiers in northern Iraq were killed when insurgents open fire as they guard oil installations near Kirkuk.