The nationwide three-hour chakka jam - blockade of national and state highways -- by farmers protesting against the Centre's farm laws started with a high alert in Delhi today. A group of farmers have blocked the highway at Haryana's Palwal. The farmers have said there will be blockade in Uttar Pradesh and Uttarakhand in case they are "called to Delhi". In Bengaluru, 30 people have been taken in preventive custody.