Blasts believed to have been caused by home-made explosives ripped through the canteens of China's two most prestigious universities within 90 minutes of each other yesterday, injuring at least nine people, according to police and university authorities. The blasts come at a politically sensitive time in Beijing as security is stepped up ahead of the opening next week of the 10th National People's Congress. It is not known whether the blasts are related to the congress. But, according to students, they have shattered the usual sense of security on the leafy Peking and Tsinghua university campuses in northwest Beijing. The bombs were home-made devices containing black gunpowder, a spokeswoman at the Beijing Municipal Public Security Bureau said. But she declined to comment on whether police investigating the blasts had any suspects or whether the two explosions were related. Speaking at a regular Foreign Ministry press briefing, spokesman Kong Quan said he was concerned about the explosions. He declined to comment further, saying he would wait for the results of the investigation. China has been rocked by a spate of explosions over the past few years, involving a wide range of culprits and suspects. They include unemployed workers, jilted lovers and Muslim separatists. Yesterday's first blast happened at 11.55am in the teacher's canteen in Tsinghua University, injuring four lecturers and two students. Jiang Yunzhong, vice-president of the university's information office, said the six had all suffered injuries to their feet. They were being treated at the Beijing University No 3 hospital. Between 30 and 40 people were in the Tsinghua canteen at the time of the explosion. Witnesses said the blast smashed the windows and light fittings. The blast in the canteen of nearby Peking University took place with fewer people around, as it went off at 1.30 pm when most students had finished their lunch. Two kitchen staff and one student from another university were slightly injured. 'It is so terrible, so terrible,' said one female student who had eaten at the canteen an hour before the blast. Another student, who was in the dormitory building 15 metres from the canteen, said: 'I just heard a big bang, and quickly realised it was a bomb. There did not appear to be many students around, mainly canteen staff. I saw a girl wounded in her leg. And I also heard a woman screaming.' By late last night, calm appeared to have returned to the campuses of both universities. Students said they saw more security guards on patrol, and university authorities had published notices saying that police would conduct security checks at bathrooms and classrooms. Peking University and Tsinghua University are located side by side in the city's northwestern Haidian district. They are widely regarded as the top universities in the country and are popular choices for students from Hong Kong and Macau. Both admit students from a wide range of countries.