Demands are increasing for tighter security in Britain after police arrested 12 mostly Pakistani nationals in connection with an alleged terrorist plot.Searches of several properties are still ongoing following the dramatic daylight arrests on Wednesday. Despite bringing the planned operation forward because of a security blunder, police believe Manchester’s main shopping districts and a nightclub were among some of the targets of the suspected al-Aaeda attacks. With the incident raising tensions between Britain and Pakistan over visa checks, the conservative opposition have called on the government to do more. Conservative home affairs spokesman Chris Grayling said: ‘‘The student visa system is being used by terrorists to try and get people into the country. If it’s being abused by terrorists groups around the world then the rules have to be much tighter. The government has been in power for a decade. There have been a number of issues around the student visa system over the years. These have been loopholes by ministers own admissions. They really need now to make sure things are absolutely water tight.’‘ Despite the suspected terrorists holding student visas, the government claims it has introduced stricter measures, such as fingerprinting, making the system more secure.