The group, organised by the Society for the Protection of Unborn Children (SPUC) Scotland, is supported by number of different faith groups, including the Muslim Council of Scotland, the Free Church of Scotland and the Catholic Church in Scotland.
Plans are that abortion law will be devolved to Holyrood under the Scotland Bill, the package of powers aimed at delivering on the recommendations of the post-referendum Smith Commission.
However opponents to the move have warned that it could undermine the consensus on the current time limit of 24 weeks in which to undergo an abortion.
However, the Scottish Government has stated that it does not have any plans to change the law.
John Deighan, chief executive of SPUC Scotland and campaign co-ordinator, said: "The rights of the unborn child are facing new threats with the impending devolution of powers to the Scottish Parliament from Westminster.
"Already, well in advance of the transfer of legislative authority, we are hearing a clamour for further extension of the existing, excessively liberal law.
"This could see abortion legalised from the existing 24-week limit, in most cases, up until the point of birth.
"In response to these deeply disconcerting demands, pro-life supporters from different backgrounds, people of faith and none, in addition to concerned organisations and individuals have decided that we must stand together in the face of this onslaught."
Free Church of Scotland Moderator Revd David Robertson said the church was happy to join an alliance "which will spread the fight for justice in a bid to influence legal attitudes to the right of life".
