1698 - Access denied for user 'root'@'localhost'

I am getting below error while logged into the phpmyadmin. it is not a fresh installation. I have also tried login through the command line.

1.mysql -u root -p
  **error :** ERROR 1698 (28000): Access denied for user 'root'@'localhost'
2.sudo mysql -u root -p

I am successfully login into mysql using above command. but not with case 1.

Mysql details:
username : root 
password : 

Решается:

I solved the above problem with setting the plugin value in user tables

mysql> USE mysql;
mysql> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
mysql> FLUSH PRIVILEGES;
mysql> exit;
$ service mysql restart