GUWAHATI: Ten days before it goes to the polls and 24 hours before Prime Minister Manmohan Singh was to land in his home state, Assam was rocked by four deadly explosions on Monday that left at least nine people dead and more than 60 people injured. ( Watch )

Undeterred by the terror attacks, the prime minister will go ahead with his visit and address a rally at Dispur on Tuesday, officials in New Delhi said. Union home minister P Chidambaram called chief minister Tarun Gogoi minutes after the blasts. Official sources said the Union cabinet secretary, defence secretary and home secretary would arrive here on Tuesday to take stock of the situation.

The finger of suspicion pointed to the northeast's most active terrorist outfit, the United Liberation Front Of Asom (Ulfa), which is to mark its 30th 'Raising Day' on Tuesday. In the past, Ulfa terrorists have triggered blasts in the state to observe the day and also planted bombs just ahead of VVIP visits, the deadliest being serial blasts in January, just ahead of Chidamabaram's first visit to Guwahati as home minister. ( Watch )

Monday's deadliest blast was triggered at a busy bus stop at Maligaon on the outskirts of Guwahati around 1.40pm and killed eight people, including a 10-year-old boy. More than 50 people were injured there. The Maligaon police station and the headquarters of the Northeast Frontier Railway are a stone's throw from the site.

According to a police officer scanning the site minutes after the blast, the explosion at Maligaon was caused by an improvised explosive device (IED) planted either on a bicycle or a motorcycle. People waiting at the bus stop were the first to be hit.

The blast was so powerful that several cars and motorcycles parked nearby caught fire. The flames soon spread to neighbouring houses. Many people jumped out of their windows in panic to escape the fire and smoke and injured themselves.

Less than two hours later, another explosion, this time at a busy marketplace in Dhekiajuli in Sonitpur district, injured three Railway Protection Force personnel on election duty. Here, too, the bomb was strapped to a parked bicycle, a vehicle of terror that has been used effectively by Ulfa terrorists.

In the third blast in the evening, terrorists threw a grenade at the Mancachar police station in Dhubri district, killing a constable. Another grenade lobbed in the Garo Basti area in Udalguri district did not explode. A fourth bomb explosion rocked Karbi Anglong district, injuring two people.

Gogoi condemned the blasts and said his government would crush the terrorist elements operating in the state.

Deputy inspector general of police (central range) G P Singh said Ulfa was behind the blasts. "We had information about the outfit's plans to carry out terror activities ahead of its foundation day on April 7," he said.

Sources said intercepted messages of Ulfa militants suggest the outfit will now target security forces and non-Assamese people.

"The Maligaon blast may have been carried out by one of Ulfa's 109th battalion members who have recently sneaked into the city. We came to know about a 27th battalion Ulfa cadre named Deepak who had supposedly hired a man called Sagar Topno to carry out the blast in Sonitpur district," a source said.

Forensic experts who visited the blast site at Maligaon said bicycle ball-bearings were packed into the bomb as shrapnel. Experts said chemical tests would be required to confirm the type of explosives used. An NSG bomb squad is also expected to arrive on Tuesday.