diff --git a/mysql-test/main/alter_table_lock.result b/mysql-test/main/alter_table_lock.result
index 620fca23315..5a787055478 100644
--- a/mysql-test/main/alter_table_lock.result
+++ b/mysql-test/main/alter_table_lock.result
@@ -14,4 +14,49 @@ ALTER TABLE t1 CHANGE COLUMN IF EXISTS b c INT;
 SET SESSION max_session_mem_used = @max_session_mem_used_save;
 UNLOCK TABLES;
 DROP TABLE t1;
+#
 # End of 10.5 tests
+#
+#
+# MDEV-28943 Online alter fails under LOCK TABLE with ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+#
+create table t1 (f int) engine=innodb;
+insert t1 values (1);
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+lock table t1 write;
+connect con1, localhost, root;
+select count(*) as 'must be 0' from t1;
+connection default;
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+delete from t1;
+unlock tables;
+connection con1;
+must be 0
+0
+connection default;
+drop table t1;
+#
+# MDEV-29056 Replica SQL thread stops with 1846 error on ALTER ONLINE after LOCK WRITE
+#
+create table t1 (c varchar(1), key (c)) engine=innodb;
+insert into t1 (c) values ('g') ;
+alter table t1 add fulltext key(c), algorithm=inplace;
+alter online table t1 add column s blob not null, algorithm=inplace;
+ERROR 0A000: LOCK=NONE is not supported. Reason: Fulltext index creation requires a lock. Try LOCK=SHARED
+lock table t1 write;
+alter online table t1 add column s blob not null, algorithm=inplace;
+ERROR 0A000: LOCK=NONE is not supported. Reason: Fulltext index creation requires a lock. Try LOCK=SHARED
+drop table t1;
+#
+# End of 10.11 tests
+#
diff --git a/mysql-test/main/alter_table_lock.test b/mysql-test/main/alter_table_lock.test
index bd26c1ac7d0..6fe717f977e 100644
--- a/mysql-test/main/alter_table_lock.test
+++ b/mysql-test/main/alter_table_lock.test
@@ -1,4 +1,5 @@
 --source include/not_msan.inc
+--source include/have_innodb.inc
 
 --echo #
 --echo # MDEV-23836: Assertion `! is_set() || m_can_overwrite_status' in
@@ -26,4 +27,54 @@ SET SESSION max_session_mem_used = @max_session_mem_used_save;
 UNLOCK TABLES;
 DROP TABLE t1;
 
+--echo #
 --echo # End of 10.5 tests
+--echo #
+
+--echo #
+--echo # MDEV-28943 Online alter fails under LOCK TABLE with ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+--echo #
+
+# test that any lock=xxx works under LOCK TABLES
+# and that it is ignored, does not actually downgrade the lock
+create table t1 (f int) engine=innodb;
+insert t1 values (1);
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+lock table t1 write;
+connect con1, localhost, root;
+--send select count(*) as 'must be 0' from t1
+--connection default
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+delete from t1;
+unlock tables;
+--connection con1
+--reap
+--connection default
+drop table t1;
+
+--echo #
+--echo # MDEV-29056 Replica SQL thread stops with 1846 error on ALTER ONLINE after LOCK WRITE
+--echo #
+create table t1 (c varchar(1), key (c)) engine=innodb;
+insert into t1 (c) values ('g') ;
+alter table t1 add fulltext key(c), algorithm=inplace;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter online table t1 add column s blob not null, algorithm=inplace;
+lock table t1 write;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter online table t1 add column s blob not null, algorithm=inplace;
+drop table t1;
+
+--echo #
+--echo # End of 10.11 tests
+--echo #
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 63d2bab583f..66a29af9caf 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -10029,7 +10029,6 @@ bool mysql_alter_table(THD *thd, const LEX_CSTRING *new_db,
   if (alter_info->requested_lock == Alter_info::ALTER_TABLE_LOCK_SHARED
       || alter_info->requested_lock > Alter_info::ALTER_TABLE_LOCK_NONE
       || alter_info->flags & ALTER_DROP_SYSTEM_VERSIONING
-      || thd->locked_tables_mode == LTM_LOCK_TABLES
       || thd->lex->sql_command == SQLCOM_OPTIMIZE
       || alter_info->algorithm(thd) > Alter_info::ALTER_TABLE_ALGORITHM_COPY)
     online= false;
@@ -10906,6 +10905,9 @@ do_continue:;
       goto err_new_table_cleanup;
     }
 
+    if (thd->locked_tables_mode == LTM_LOCK_TABLES)
+      online= false;
+
     // If EXCLUSIVE lock is requested, upgrade already.
     if (alter_info->requested_lock == Alter_info::ALTER_TABLE_LOCK_EXCLUSIVE &&
         wait_while_table_is_used(thd, table, HA_EXTRA_FORCE_REOPEN))