Two letter bombs burst in separate places in Thiruvananthapuram on Thursday, one of them in a post office while a crude bomb went off in a nearby rural area, hours before President A P J Abdul Kalam's visit to Thiruvananthapuram.

No one was injured in the blasts and police said the sealed covers contained only "cracker material" in both the cases.

The first incident occurred at the Manacaud post office when an employee was sorting postal articles. Two other sealed covers were also found containing some explosive materials, which were later taken away by the bomb squad for defusing, police said.

The explosion took place when the sorting clerk pressed a portion of the cover, which was found slightly projected.

The clerk became suspicious as the cover was found to be slightly heavier than normal envelopes.

Later in the day, another letter bomb burst when a shopowner opened a cover at Kesavadaspuram in Thiruvananthapuram. All the covers were posted from post offices in Thiruvananthapuram.

However, police was yet to establish whether the addresses to which they had been sent were genuine or not.

Meanwhile, one person suffered minor injuries when a country-made bomb exploded in an agricultural plot near Malayinkil in Thiruvananthapuram suburbs, police said.

DGP Raman Srivastava said the IG, South Zone, had been asked to investigate the case. Security has been tightened in Thiruvananthapuram in view of the President's visit.