SQL Server 2005 to 2012 Replication, cannot access replicated data
This is covered in Frequently Asked Questions for Replication Administrators:


  Are tables locked during snapshot generation?
  
  The length of time that the locks are taken depends on the type of
  replication used:

For merge publications, the Snapshot Agent does not take any locks.

For transactional publications, by default the Snapshot Agent takes locks only during the initial phase of snapshot generation.

For snapshot publications the Snapshot Agent takes locks during the entire snapshot generation process.

  
  Because locks prevent other users from updating the tables, the
  Snapshot Agent should be scheduled to execute during periods of lower
  activity on the database, especially for snapshot publications.