This weekend saw millions of Muslims mark the day of Ashura around the world.  In central London, about 3,000 Shi'a Muslims gathered to commemorate the martyrdom of Hussain Ibn Ali, the grandson of the Islamic prophet Muhammad. He and a small group of followers were slaughtered at the battle of Karbala in Iraq, by the Arab rulers of 1400 years ago.  Many Shia's use the day to commemorate his life as well as focusing on current issues of concern -- as Catherine Drew reports from the British capital.

Ashura is commemorated throughout the Islamic world.  But the day is of particular significance to Shi'a Muslims who remember the martyrdom of Hussain Ibn Ali, the grandson of the Prophet Muhammad and the man they consider to be his rightful successor.

Ashura means 10th, it falls on the 10th day of Muharram, the first month in the Islamic calendar.

Those marking the day wear mourning clothes and slap themselves as an expression of grief.

Participants say the day is dedicated to remembering Hussain's heroic struggle to keep the faith from corruption, his battle against injustice and tyranny, and how that relates to the present.

One participant said, "Those things are still happening today, so that is why we try to remember what has happened even four centuries ago and why have not we changed."

Another participant said, "In the modern time, the main point, people get back, you know, to  work every day, back to religion and what we are doing now and try to be a better person, to be honest."

Yet another participant said, "What I can learn is peace; what I can learn is good conduct; what I can learn is [to be] good with people.  It does not matter where they are from because at this time, when he was ruling, you had Muslims and non-Muslims, you had Jews and Christians living in peace."

Many Shi'a relate Hussain's struggle against injustice to current issues.

The Ahl Al-Bayt society, which is named after the Prophet Muhammad's household, at Imperial College London used the start of the month of Muharram to highlight the shortage of clean water in the developing  world.

Society President Wasim Ukra says lessons can be learned from the martyrdom of Hussein. "Imman Hussain, who is the grandson of the Holy Prophet, peace be upon them, stood in the plains of Karbala in the desert fighting for this cause, and that is one of justice and human values.  He was denied this right, which was access to water.  Not just himself, but the women and the children, including up to two-month old babies.  So we can directly relate to this, the suffering they went through and relating to that to today, seeing what is happening in the world today."

Ashura commemorations have been marked by violence in the past, in Iraq and Pakistan. However, Shi'a living in Britain were able to march in peace.