A Study Evaluating Safety and Tolerability of YM150 Compared to Warfarin in Subjects With Atrial Fibrillation

Inclusion Criteria: * Subject with paroxysmal, permanent or persistent Nonvalvular Atrial Fibrillation (NVAF) * Subject has prothrombin time international normalized ratio (INR) of 2.0 or below at the baseline visit Exclusion Criteria: * Subject has current or recent (within 12 months prior to screening) history of stroke and/or systemic embolism (including TIA) * Subject has active bleeding or any condition associated with increased risk of bleeding * Subject has NVAF secondary to other reversible disorders (e.g. thyrotoxicosis) * Subject has an indication for warfarin other than AF (including planned cardioversion) * Subject has had a diagnosis of Acute Coronary Syndrome (ACS), coronary artery bypass graft (CABG) or percutaneous transluminal coronary angioplasty (PTCA) within previous 3 months of screening * Subject has a diagnosis of left ventricular aneurysm or atrial myxoma * Subject requires use of prohibited previous and concomitant medication (i.e., thrombolytics \[such as tissue Plasminogen Activator (tPA), streptokinase\], antiplatelet agents \[such as cilostazol, clopidogrel, ticlopidine, dipyridamole\], anticoagulants \[such as vitamin K antagonists, heparin, unfractionated or low molecular weight heparin, fondaparinux, thrombin inhibitors\] and chronic use of nonsteroidal anti-inflammatory drugs \[NSAIDs\] or acetylsalicylic acid use of \>100 mg/day). (see Section 5.1.3 and Appendix 1 for details) * Subject has active infective endocarditis * Subject is planned for invasive procedures with potential for bleeding * Subject has participated in another clinical trial of an investigational drug (including placebo) or device within 30 days (or the limit set by national law, whichever is longer) of signing informed consent for the present study * Subject has participated in any YM150 clinical trials