public function login() {

    if ($this->request->is('post')) {
        if ($this->Auth->login()) {
            return $this->redirect();
        }
        $this->Session->setFlash('Incorrect user or password.');
    }
}

<h1>Access with your username:</h1>
<?php
    echo $this->Form->create('User', array('action' => 'login'));
        echo $this->Form->input('username', array('label' => 'User:'));
        echo $this->Form->input('password', array('label' => 'Password:'));
    echo $this->Form->end('Login');
?>

<div>
    <?php
        echo $this->Form->create('user', array('action' => 'login'));
        echo $this->Form->input('username', array('placeholder' => 'User', 'label' => false));
        echo $this->Form->input('password', array('placeholder' => 'Password', 'label' => false));
        echo $this->Form->submit('Ingresar', array('div' => true));
        echo $this->Form->end();
    ?>
</div>