Security was beefed up and cellular and internet services suspended in major cities of Pakistan on Tuesday to maintain law and order during the observance of Chehlum.
Processions will be held in different parts of the country by Shiite Muslims and the government has deployed thousands of troops along the routes and around the venues where the marches will terminate by the evening, officials said.
Chehlum is observed to commemorate the 40th day of the martyrdom of Imam Hussain, the grandson of the Prophet, in 680 in Karbala in modern Iraq.
Officials said the security was tightened to avert any tragedy on the occasion of the Chehlum.
Extremist Sunni groups in the past have attacked such rallies with suicide bombings.
Earlier, interior ministry received applications from home departments of all the four provinces to suspend the mobile phone and internet services, the officials said.
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Local TV reports showed major cities like Lahore, Karachi, Peshawar, Quetta, Rawalpindi, Muzaffarabad, Multan facing complete or partial suspension of services.
Modern communication services are used by terrorist to plan and carry out acts of terrorism, security officials said.