If the transaction is open, It wont allow you to even select records (Which requires shared lock only)from that table from another session. Update is out of question.
create table tabb1 (c1 int, c2 int)
insert into tabb1 values (1,1)
insert into tabb1 values (2,2)
begin transaction tr1
update tabb1 set c2=11 where c1=1
Try it out.